Cannot execute as the database principal because the principal "xxxx_maskedusername" does not exist, this type of principal cannot be impersonated, or you do not have permission.
However, with that said. I logged in as the client with SSMS and was able to run the script and only got a single error and a warning this time. The site is upgraded and running yay!
(304 rows affected)
(168 rows affected) Caution: Changing any part of an object name could break scripts and stored procedures.
Msg 4902, Level 16, State 1, Line 3241 Cannot find the object "dbo.RewardPointsHistory" because it does not exist or you do not have permissions.
This is the only reference to RewardPointsHistory within the entire script. I would try removing the dbo. (just after ALTER TABLE), and run this particular snippet again (under the context of your user)
--new column IF NOT EXISTS (SELECT 1 FROM sys.columns WHERE object_id=object_id('[RewardPointsHistory]') and NAME='UsedWithOrder') BEGIN ALTER TABLE dbo.RewardPointsHistory ADD UsedWithOrder uniqueidentifier NULL END GO
This is the only reference to RewardPointsHistory within the entire script. I would try removing the dbo. (just after ALTER TABLE), and run this particular snippet again (under the context of your user)
--new column IF NOT EXISTS (SELECT 1 FROM sys.columns WHERE object_id=object_id('[RewardPointsHistory]') and NAME='UsedWithOrder') BEGIN ALTER TABLE dbo.RewardPointsHistory ADD UsedWithOrder uniqueidentifier NULL END GO