Goonie, now that I have the 3.15 release on my phone, can I be sure that new transactions will not send change back to any of the old addresses that were generated by pre-3.15 versions?
Yes, you're safe. Insecure addresses are excluded from any of the internal operations, except for still showing them in the address list for reference.
By the way: Even if you receive more coins to one of the insecure addresses (which you should try to avoid), a new key rotate transaction will be created for that amount.