An end user could almost always use an older version and still be fine. Processors rarely make changes that cause older api's to not work. More frequently the transactions processed using an older api do not qualify for the best rates. When an upgrade is required, the merchant's bank will usually tell the merchant they have to be using the most recent version, or they can't process transactions.
And so the question that the bean counters have to answer is "which would cost us more: the upgrade or the lost revenue due to having the old version?". Sometimes, it just doesn't pay to upgrade.
thor
Feel the white light, the light within
Be your own disciple, fan the sparks of will
For all of us waiting, your kingdom will come