I'm trying to work out if it's possible to migrate workflows that use default client events that are internet web page URL's without having to open the workflow in studio and change all the reference before the export?
The easiest way to do this would be to save you internet URL's as String Table entries. Instead of hard coding a URL into the Default Client Event, rather use the String Table variable. The String Table Entry can then be edited/changed through K2.net Service Manager after it has been exported. In this way, you would not need to reopen K2.net Studio and re-export your process.

Also, is the studio export the only way to do the export to a new server? You can have multiple export servers, but can you have different client event properties for each server!?
Except for String Table entries, you can not have different client event properties for each server. You can also write a custom app to do the export but you still need to use the K2Studio object model.

Hope this helps,
Ockert