Note that it is best to make a direct trustee to trustee transfer to a rollover IRA or other 401k, so there is no withholding.
If you have a check made out to you, there is manditory 20% withholding. You have 60 days to roll that over, plus the missing 20% to avoid tax or penalty. You would get credit for the 20% withheld at tax time. But if you cannot come up with the missing 20% for the 60 day rollover, that 20% would be taxed (and 10% penalty if it applies).
I don't know how long they have to distribute the money to you from the old 401k plan. But when our company changed trustees, the old trustee held it in a money market account for a full year before we could do anything with it. With our current plan, if we leave the company, we have a limited time to determine what to do with it, or they will send us a check (likely short the 20% withholding). So it would be in your best interests to contact the administrator of your old 401k plan to see if there is a time limit for you to make a decision and arrangements for a trustee to trustee transfer. Definitely make sure that they have your current address.