Alright good news, even though it requires an upgrade for the users, good news, I debugged the exec podcupdate feature this morning and I did find a bug! This explains why T-Mike cant send the exact right stake amount and Simons UTXO is 0 and Rastiks is probably the same issue.
So I now confirm you dont need to worry about the source address of the funds, we will still sign the UTXO packet with your CPID no matter which "bucket" it originates from in the wallet, so that will alleviate a huge portion of the hassle with sending a PODC Update (in the next version). The other biggie I found was the math the wallet was using was slightly off due to rounding, its kind of hard to explain, but the lions share of the problem was, if you were trying to send say 49K of your 50,000 balance, and your tasks consumed say 3.5K bytes, the internal math (in decimal) rounded down to an amount that was not properly split among the vouts ending with too small of a UTXO expenditure, so I had to fix that so the PODC update leg count (internally) would always round up the specific UTXO for that amount per leg, its a little technical but that is now fixed (in the next version).
This should be checked in ASAP, doing a little unit testing with this.