How do I stop the app from carrying over leftover money in the envelopes from one month to the next?
At the beginning of the month, do a Fill using the “Set” option for every envelope and enter the amount you’d like it to show (make this recurring to be easier). If you don’t want ANY money in the envelope on the first, then Set every envelope to 0 (but you’ll have to then distribute it all appropriately). Most people have their monthly Fill marked to Set all the envelopes that don’t change much, and Add for the discretionary ones.
Is it possible to change an envelope from Add to Set mid month, or can this only be done when filling at the start of the month?
You can change future transactions at any time, but if you need to adjust the balance of an envelope for a Fill that’s already posted, just do an envelope transfer to/from Available (or any other envelope) which will have the same effect.