Well, I may not be doing this correctly, but it's always worked before:

We have someone who opted to change their type of membership. We don't allow users to upgrade themselves, and this is more of a "lateral" anyway. I deleted the user's current subscription in the back end and attempted to add the new one. The user record is saved, but no subscription is added. A check of CBSubs-->Subscriptions confirms there is no subscription in the database for this user. I cleared the cache and used three browsers to no effect.

Thanks for the help!

Thanks, Kyle. Nope, it wasn't a lifetime plan. What finally seemed to work was to clear the "Supercacher" cache at Siteground. I'm not sure why this is a cache issue, but I'll take success any way I can get it.

That's interesting and very informative. I thought this particular caching option only cached static data, since Siteground also offers a "Dynamic Cache" in coordination with a Siteground Joomla plugin that in my case really screws things up. It sounds like I should turn off the "static" cache, too.