[wp-hackers] notice before plugin upgrade

[Rich Pedley]

I realise this is only suitable for automatic upgrades but is there 
any method that I can hook into the upgrade routine for plugins with a 
custom message prior to the automatic download/upgrade?

I need to make changes to a core file of my plugin and want everyone 
to have the chance to _not_ upgrade to it. I intend doing this in a 
few versions time, so want to put the code in place _now_ to try and 
catch as many people as possible.

Obviously I can't do anything for anyone manually upgrading.

Thoughts and ideas welcomed.

Rich