To add to this, my last 7 reholders all kept their cert numbers. One was a signature series black cat campbell signed virgin 601 homage cover.
Had 2 venom lethal protectors, an asm 606 for nyc spidey label, 2 facsimile (asm 1 and asm annual 1) for the nyc spidey label and wolverine origins #1 for the wolvie label....
Just verified the submission forms with the relabel numbers and the what came back matches.
My xmen #1 claremont sig had to go back with ME.. it kept its number too, allowed me to notice they missed my custom label and get money back for that.
As far as ive seen, it only gets a new number if it goes through regrading.