Your trigger would have to look at Record Type changes. I suppose you could run a nightly batch to capture this case if it's not urgent.
You should test this.
Thanks Josh - I guess my question is more around, if the merge and record type changes in SFDC - what happens in Marketo? Do those leads stay in the partition that they originated in, and if there are two duplicate leads (created from SFDC) in two separate partitions, what happens then? I'm assuming that they'd just remove from those partitions and land in default. Any thoughts?
Leads will stay in whatever partition they're currently in UNTIL something happens that triggers a campaign to move them elsewhere. Our partitions are country-based. So if a record exists in the US partition and another in the Canada partition (highly unlikely, I know) and if we merged the Canadian record into the US record in CRM, the lead would remain in the US partition (and there would be no lead in the Canadian partition). And of course, the lead would also exist in the default partition.