Here's a list of the changes made to existing instruction in Version 5 of RATS. See the Version 5 page for general information on the new release of RATS.

ALLOCATE	The little–used parameters equations and maxvar have been replaced with options of the same names.
AR1	Hildreth–Lu (HILU) is now the default method, changed from Cochrane–Orcutt (CORC). In addition, you now have the option of inputting a value for rho, rather than having it estimated.
BOOT	Added [REPLACE]/NOREPLACE option to allow drawing without replacement.
CALENDAR	The new RECALL option and the %CALENDAR() function allow you to temporarily change and then restore your CALENDAR settings. There are new date and time functions as well.
COPY	New formats: HTML, CDF (comma-delimited format), plus the new PICTURE option for controlling format of output. We now use the COLUMNS/ROWS terminology for the ORG option, although the older ORG=VAR/OBS terminology is still supported.
CORRELATE	New METHOD option selects between the new Burg algorithm (the default) or the Yule–Walker algorithm used in earlier versions.
DATA	Excel format has been updated to allow reading files created by more recent versions. CDF (comma-delimited) is now accepted. ORG=COLUMNS/ROWS has superseded ORG=VAR/OBS, though the latter are still supported.
DISPLAY	Many new capabilities for formatting output, including new picture and position code options, and the ability to display entire arrays (previously you had to use WRITE for displaying entire arrays). The new %BESTREP and %MINIMALREP functions are also useful with DISPLAY.
ENVIRONMENT	The new GSAVE and GFORMAT parameters allow you to automate the process of saving a series of graphs into separate files in any of the file formats supported by rats.
EQUATION	New COEFFS option allows you to assign coefficients for identities as you define the equation (rather than using ASSOCIATE later). The ARlags and MAlags parameters now accept lists of lags.
ESTIMATE	New options COEFFS and RESIDUALS provide an easier way to save estimated coefficients and residuals. COHISTORY option allows you to save the coefficient history when doing Kalman filtering. VAR coefficient sums are now stored in the array %VARLAGSUMS and %SIGMA is defined as the covariance matrix of residuals.
EWISE	The new %DO function is sometimes a better alternative to EWISE—see example under EWISE.
FIND	Now offers GENETIC and BFGS optimization methods in addition to SIMPLEX. New ROOT setting for finding a root, plus new options for controlling output (TRACE, NOPRINT, etc.). Slight syntax change, with the use of END FIND to end the controlled instructions. FIND can now control other non-linear estimation instructions.
FORECAST	New RESULTS option simplifies saving forecasts for multi-equation systems. Models can now contain EQUATIONS as well as FRMLs, including combinations of both in a single MODEL. WINDOW option allows you to direct output to a spreadsheet-style window.
FRML	New PARMSET option for creating or modifying a parameter set when using the EQUATION or LASTREG options to construct the FRML. The VECTOR option allows parameters pulled from the model to form a vector. You can now declare FRMLs of types other than REAL—a FRML of VECTORS for example.
HISTORY	New RESULTS options simplifies saving results into series. New LABELS option allows user-defined labels for the decomposition results. Now supports MODEL option (can only be used with MODEL’s of equations—FRMLs not allowed).
IMPULSE	New RESULTS and LABELS options (see HISTORY above).
INQUIRE	New functions augment/supersede some capabilities of INQUIRE (see description of INQUIRE for a list).
KALMAN	Now includes RESIDUALS, COEFFS, and COHISTORY options to make it easier to save residuals and coefficients from multiple equation models, and the “history” of coefficients over time for single-equation models. Note: The RESIDS option used previously to select the method of computing residuals has been renamed as RTYPE.
MAXIMIZE	New GENETIC method supported, and the new PARMSET operation allows use of parameter sets.
NLLS	New PARMSET and STARTUP options for selecting parameter sets and providing a separate formula to be evaluated for the first entry in the estimation range. SIMPLEX and GENETIC algorithms now offered in addition to the default Gauss–Newton method.
NLPAR	New EXACTLINESEARCH option, plus options for controlling the new genetic algorithm.
NNLEARN	New ITERATIONS option provides additional control over the model fitting process.
NNTEST	New VALIDATE option for computing a mean squared error from comparison with target output set.
NONLIN	Now allows arrays as well as scalar reals as parameters and allows for “substitution” constraints and equality and inequality constraints for doing constrained optimization. Also, now supports multiple parameter sets via the PARMSET option.
OPEN	New WINDOW option.
ORDER	New %SORT and %RANK functions provide alternatives to ORDER
OVERLAY	New matrix extraction functions %XSUBMAT and %XSUBVEC (plus %XDIAG, %XROW, %XCOL that were added with 4.2) provide a useful alternative to OVERLAY in certain circumstances
PANEL	ENTRY, INDIV, and TIME settings are now handled as options rather than parameters (although the parameter method is still supported). The new PREGRESS instruction provides an alternative method of doing panel data regressions.
PRINT	New PICTURE and WIDTH options for controlling output formatting, and a WINDOW option for directing output to a spreadsheet window.
PRTDATA	New formats, including HTML and CDF are now available
PSTATS	Can now handle unbalanced panel data sets.
SCATTER	Features many new options, including the ability to do overlay (two-scale) graphs, log-scale graphs, shading, and more.
SIMULATE	New RESULTS option for conveniently saving simulation results, and a new DECOMP option for supplying your own decompositions.
STATISTICS	The MOMENTS option now computes the Jarque–Bera normality test.
STEPS	Same changes as FORECAST (see above).
SYSTEM	The new MODEL option allows you to define the system as a MODEL, rather than a list of equations, which simplifies the use of instructions like FORECAST, IMPULSE, HISTORY.
TABLE	New WINDOW option directs output to a spreadsheet-style window
WRITE	The DISPLAY instruction has been upgraded to handle most tasks which used to required WRITE.