Upgrading DB apps

[Simon Wistow]

I have a app that relies on you having installed it and then run 
setup to create a table in a database. In fact the app has several 
plugins and each plugin has at least one database table. 


What I'd like to do is make it so that if you upgrade to a newer version 
of a plugin then it migrates your data from an earlier version of the 
schema to the latest version.  

Since plugins may be distributed seperately from the main app I'd like 
to make the upgrade mechanism available to all plugins - external or 
'core'.

Is there a sane known way to do this?

Would the requirement that you can't upgrade more than one step at a 
time (like for example requiring that you apply patches in order) be too 
onerous a restriction (i.e you can't jump straight from 1.0 to 3.0 you 
must go 1.0 -> 2.0 -> 3.0)

Simon