Until last month I had just one country in my SFDC.
Now we implemented for more 12 countries and all Leads were synced with my default Lead Partition before I setup "Assignment Rules" for my old and default partitions.
If I setup it now say that just Lead from my country is allowed, what will happen? Leads from a will be removed from partition?
Many issues there:
Like Greg mentions, it's a best practice to just ignore the regular assignment rules and create the appropriate programs/campaigns to ensure all scenarios are addressed (route existing, modified and new leads).
Within these campaigns we not only have rules (choices) setup to do the proper assignment, but we also take the opportunity to normalize the country name. For example, our "list acquisition" excel template uses the 2-character country code. When lists are imported into Marketo, those codes are normalized to the full country name. Same thing for "inferred country" when new leads enter our system.
I personally prefer to separate lead routing from data quality programs
If necessary, you can set is a way that the data quality changes trigger the lead routing ones. For instance:
Data Value Changes (filter: country = bad value) -> clean the country value (change data value of Country to correct value)
Data Value Changes (filter: Country = Correct Value) -> route the lead.
Makes sense - and typically we do the same. But for this, we just combined in under a master set of programs for both country routing and normalization. Here's our NA campaign: