How to take a pre-payment for customer before move-in

If I have an existing tenant, and I held a unit on reservation, in Leads to Lease, for a second unit and now I need to install the new move in, how can I be sure that his current unit is related with this new unit?

@jcatt - Just in case someone is searching this info, as @skinman said - enter their name and search - when the list of customers with name comes up, choose the current customer, double click and always choose NO - this will create a new reservation with all the current customer's info populated automatically.  It will add the new unit to their current account when they move in.  
Sometimes when a current customer makes an online reservation which then is NOT linked to their current account, I have to go in and redo the whole thing so they link.
@cgothard - You can do a premove in by changing the move in date under Starts in the Move In window.  Unfortunately, this is a set in stone thing.  They will not be able to access the gate sooner than that if they change their mind.  I guess you could let them in manually during office hours.
I agree with @jcatt in that it is risky and they could come back and say, "Nevermind" and want a refund.  This would create undo work for us, so we try not to take money until the actual move in.