The correct way is to pay contract for NMS-BASE-250 till the date of purchase of upgrade. Once you purchase the upgrade you can buy the contract for NMS-250.
The team taking care about services should not accept contract for upgrade if the base is not covered.
There can be issue in consuming the contract if it is not correct = there is only upgrade covered but not the base.
Regards
Z.
Regards
Zdeněk Pala