RFL Rebanding advice

Goaty:
A friend of mine has bought himself a toy. A big one with a V8 in it. His intention is to refurbish it and take it to truck shows. The chap he bought from used it on stepframes doing abnormal work on STGO cat 2. As such it’s currently banded as “Special Types”. He wants to re band it to PLG, £165 (ish) per year RFL.
A short and frustrating conversation yesterday with a young lady at the DVLA was of no use at all. She was adamant it’ll have to remain banded as ST, as PLG is only for vehicles up to 3.5tn. I’m sure this isn’t the case but I’ve been out of the loop for a couple of years now and would appreciate some guidance from a grown up. :smiley:

I know that tractor units can still be taxed as PLG.
I don`t know anything about changing from one group to another however.