It seems Microsoft just adds all purchased licenses to a total count without distinguishing between resellers.
Then users just use a license from the total count.
I recently quit using a reseller for my 'Microsoft 365 Business Standard' licenses and instead directly ordered the same amount of licenses via Microsofts site (was not happy with the reseller). So for a short period I had a double amount (from both reseller and Microsoft).
Basically then the new licences seemed just to add to the number of available licenses.
And when the licenses expired from the reseller, it just lowered the total amount of licenses and the users kept working without changing any settings.
Then users just use a license from the total count.
I recently quit using a reseller for my 'Microsoft 365 Business Standard' licenses and instead directly ordered the same amount of licenses via Microsofts site (was not happy with the reseller). So for a short period I had a double amount (from both reseller and Microsoft).
Basically then the new licences seemed just to add to the number of available licenses.
And when the licenses expired from the reseller, it just lowered the total amount of licenses and the users kept working without changing any settings.