register_deactivation_hook() question not found in the codex

hello there,

i have a premium wordpress plugin that i am trying to enable auto updating of.

i dont need to hook into the core wordpress update class as i need to get user information (client login) details to authorize the update.

i was thinking i could:

1.run a check for a new version
2. if there is display a “update” page in the plugins menu
3. when they provide there details run the upgrade

the upgrade would work like this:

1.check client details through http
2. if okay deliver zip file
3. unzip file and move to plugins folder
4. deactivate current version
5. activate new version

all of these steps i know how to achieve on there own, but what i need to know is if i will get “function cannot be redeclared” when i try to activate the new plugin.

if i run a function from “register_deactivation_hook” will the plugin be deactivated BEFORE or AFTER it runs the function.

if it after i will be okay, but if its before i need a new solution