When bringing an old mission up to version 1.x.x, you will encounter the following error:

----------------------------------------------------------------------------------------------------------------------------------
https://imgur.com/a/MqlLhtq

Attention! This mission was previously saved using the following 3DEN Editor mod(s):

MEH_MPSYNC
MEH_COMMUNICATIONJAMMER
MEH_ANTITROLL

You won't be able to edit or save any custom attributes added by the mod(s). End now and install the missing mod(s) to prevent any loss of custom data.
----------------------------------------------------------------------------------------------------------------------------------

This error pops up because these addons are no longer a part of the mod. They are merged into MEH_MODULES. You can safely close this dialog, save, then check all of your modules as there may be settings lost in the port.

If you need to go back to pre v1.0 due to additional unforeseen issues, here is the link to the latest pre v1.0[github.com].

Also, did you know that you can select everything in your mission, hit Ctrl+C, open a new mission, and Ctrl+V? Try that if your mission becomes corrupt during the port.