What kind of a Local does not let you dovetail your seniority? You carry your seniority no matter what job you bid. I have changed jobs many times and I always have the same seniority date.
Part-time usually doesn't have classification seniority.
When BUG and everyone else are badmouthing part-timers, and pressuring them to go full-time, no one mentioned to them that they will:
A) Loose their seniority, and be the lowest full-time
And
B) have to start progression over from the bottom pay rate.
Full-time & part-time progressions are separate.
Unless they make sure they are going to be red circled before changing classification. Change jobs and don't grieve the lower pay in a timely manner, and you will be at the bottom tiers of pay for years.