I am trying to import a 2021a model to a previous year model so that I can eventually run with OPAL-RT. I tried 2020b and 2019a by save -> import to previous year, but for both, the blocks are 'not found' in the previous versions. These are simple blocks such as powergui, three phase AC source etc which are present in my 2020b simulink installation if I add them directly. 
The error message in my 2021a command promt when I try the import is below. It is followed by the 'ver' command from 2021a and 2020b, respectively, for confirmation of successful installation of required libraries. Also a snapshot of the unsuccessfully imported file in 2020b.
Also note that some of the examples from which my model is made can run successfully when I open them from 2020b command prompt (eg power_OLTCregtransformer), but my model can't.
How can I fix this?
Error: Export to previous version does not support Specialized Power Systems blocks from Simscape Electrical. (physmod:powersys:library:ExportToPreviousVersion)
    C:\Program Files\MATLAB\R2021a\toolbox\physmod\powersys\library\+slexportprevious\+preprocess\SimscapeElectricalSPSBlocks.m,13
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+slexportprevious\+internal\CallbackDispatcher.p >CallbackDispatcher.runCallbacks,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+slexportprevious\+internal\@PreprocessHelper\PreprocessHelper.p >PreprocessHelper.runPreprocessFunctions,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+slexportprevious\+internal\@PreprocessHelper\PreprocessHelper.p >PreprocessHelper.doPreprocess,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+slexportprevious\+internal\@PreprocessHelper\PreprocessHelper.p >PreprocessHelper.run,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+slexportprevious\+internal\@ExportController\run.p >i_run,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+slexportprevious\+internal\@ExportController\run.p,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+slexportprevious\+internal\exportToVersion.p,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\slexportprevious\+Simulink\exportToVersion.p,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\core\general\+Simulink\ExportModel.p,0
    C:\Program Files\MATLAB\R2021a\toolbox\simulink\ui\studio\config\m\+SLStudio\FileMenu.p,0
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ver command
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MATLAB Version: 9.10.0.1602886 (R2021a)
MATLAB License Number: 621625
Operating System: Microsoft Windows 10 Enterprise Version 10.0 (Build 18363)
Java Version: Java 1.8.0_202-b08 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode
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MATLAB                                                Version 9.10        (R2021a)
Simulink                                              Version 10.3        (R2021a)
Deep Learning Toolbox                                 Version 14.2        (R2021a)
Reinforcement Learning Toolbox                        Version 2.0         (R2021a)
Simscape                                              Version 5.1         (R2021a)
Simscape Electrical                                   Version 7.5         (R2021a)
Statistics and Machine Learning Toolbox               Version 12.1        (R2021a)
Symbolic Math Toolbox                                 Version 8.7         (R2021a)
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MATLAB Version: 9.9.0.1592791 (R2020b) Update 5
MATLAB License Number: 621625
Operating System: Microsoft Windows 10 Enterprise Version 10.0 (Build 18363)
Java Version: Java 1.8.0_202-b08 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode
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MATLAB                                                Version 9.9         (R2020b)
Simulink                                              Version 10.2        (R2020b)
Simscape                                              Version 5.0         (R2020b)
Simscape Electrical                                   Version 7.4         (R2020b)