After my return has been submitted, can I change things in the software to see different scenarios? For example, see the effect of filing separately instead of jointly.

You can’t change your return once it’s submitted and accepted. You can amend it, however. 

Yeah, I get I can't change my return without doing an amended return.

I just want to run some "what ifs" with my current data.  Assuming I get a favorable result, I'll then amend my return.  I don't want to setup a dummy account and enter all my data back in, I just want to run scenarios like filing separately vs. jointly.