don't quote me on this but... savings credits are a result of your subscription, and checking credits are a result of the outright purchase of credits ...or maybe it's the other way around
if you have subscription credits available and you let your subscription expire, you can't use those credits. i think those credits are put on hold until you renew your subscription.
but if you have a subscription, and purchase credits, if/when your subscription expires you still have the purchased credits to use. i hope that makes sense...
and for a "creative" way to get credits... how well can you beg?