Did you have a single or return?
The correct way to do such an upgrade is from Ludlow to Reading not from Newport. You cannot partially upgrade a ticket half way along the route in the way you describe.
So the upgrade from a super off peak return to offpeak first return would be from 51.70 to 138.00. This would come out at 86.30 which is not too far off from the stated 100 from the
TM▸ .
If however you had a single the upgrade from super off peak single to anytime first single would be from 30 to 117. This would be 87.00.
What I am describing is the correct way to do it as staff should be trained to do. Of course you could just charge the difference between a standard single and a first single and that would be about 38 quid.
However that is NOT the correct way to do it but it is quicker so may be done sometimes. Especially if a customer opens their wallet and shows about half dozen such ticket already issued before.