I made many decks and cards on my windows PC, however I tried sync them to my MacBook and it said they were too different and so all my cards were deleted/lost. How can I prevent this from happening, I’m not switching devices again but just incase something goes wrong with my Mac.
Also I optionally chose to download an older version of Anki to support more Addons, could this cause me any issues? Any help would be appreciated 
the only way to prevent this is to carefully read warnings and not just press ok without reading the warning.
but if you have lost your cards and want to recover them, you can go to file -> switch profile -> open backup and then recover your collection from a local backup. (on the older pc)
then to have your cards back on AnkiWeb, press sync and this time, press upload when the pop up pops up.
then to have your cards on the new pc, press sync on anki that’s installed on the new pc and this time on the pop up, press download.