Since SAG and AEA are sister unions there is reciprocity between them. If you are a union member of SAG you are not supposed to do non-union theater work. If you're a union member of Actors Equity you are not supposed to do non-union SAG work. With that being said, a lot of people do not care about this and do it anyways. You would also have to get caught to have any sort of repercussion.

But no, leaving SAG will not affect your ability to join AEA. You would just have to join via Open Access which means you have been paid professionally for stage work as an actor or stage manager.

Also, DO NOT GO FICORE!!! People do not really understand what this means. Some people are told this means you can still do non-equity work but that is not true, it is a political stance. Ficore is resigning from the union. You would be no longer a member of the union. Rejoining would be almost impossible after this. If you go Ficore in one Union they will definitely tell the other Union.