Upgrading with each release means fewer "issues" each time. Waiting longer in between can make it a little more cumbersome.
It's always advisable to set up a temporary test site for practicing the upgrade on, just to be sure you can sort out all "issues" without risk to your live site. Then once you've got things sorted out to your satisfaction then you can apply the changes to your live site. I wrote about the process here:
http://www.zen-cart.com/entry.php?3-...d-of-upgrading
Of course, upgrading is a desirable end result, but in the interim the suggestions I posted above will probably help resolve the immediate pain point of sudden problems while processing payments.
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