An upgrade means they are junior and new, not a senior mechanic.
Maybe you need to learn to read your IAM/US CBA, it is not DOH when you entire a new classification, you get a classification date, which is the first day you start in the new classification.
The classification date is used for bidding shift, location and RDOs.
There are people that have DOH, for travel, utility and/or stores dates and a mechanic classification date.
Your Utility nor Stores classification dates do not carryover if you upgrade to mechanic, never has been done.
How long have you worked in Maintenance and you don't even know that?
Maybe you need to sit down and read your contract, it seems I know it and you clearly don't.
Or do I need to give you lessons since you don't know how seniority, upgrades and classification dates work?