Will I lose my slot on my PCS orders if I get promoted to E-5 before I PCS?

There is a high probability of that happening because as an NCO HRC usually won’t send you to an assignment to a lower grade. So it depends on when you get promoted. Normally when you are promotable and your receive assignment instructions, it’s usually in your promotion grade. I highly recommend you call your branch manager to inquire about it, doing this may help you to get a new assignment as an E5 to the same location. Good luck to you....

That depends on the slot that you were locked into. If you were promotable when you replace on assignment, then you were placed onto an E-5 slot
If you requested the assignment through reenlistment, and you were a non promotable E4 at the time you requested it, HRC will attempt to put you into an E-5 slot at the same post.

AR 614-200 Chapter 3-2 Block G States the following:
g. Promotion of Soldiers to MSG and below will not be the sole reason for a PCS. However, Soldiers selected for promotion will be considered for assignments in their promotable grade.

So if you reenlisted for the assignment or came down on assignment instructions while promotable they projected you for your promotable grade (assuming e5 in this case). If you attained the promotable/promotion after than you should see your career counselor or call your branch manager.