Site move and development question

Permalink
I manage a working site in addition to my own, using C5 version 5.5 or some-such. Specific version not important for this purpose. The question is this: the active site is using a different hosting service and a different community user name than mine. Still not a problem, but the development site (C5 version 5.7) and the eventual host for their active site needs to be under control of the current site manager. Me or whoever replaces me. Note that I have seen the "Re-connecting to the community" discussion. I can set the set the development site up under a new community member name, and will need to do so to buy theme and plugins. When their dot com moves, that needs to happen quickly and smoothly. What difficulties might I need to plan for? Am I missing anything here when I move their dot com?