backup ER-3 and restore to ER-5

Hey all,

quick question that I think will work as I expect...but wanted to be sure.

I have a customer that has 3 WAN connections available to them now, and I want to use Primary and two failover.  In the event primary goes down, it'll roll to the 2nd, and if that's down roll to the 3rd.

I was going to purchase a ER-5 for them, and just wanted to make sure that my thought of backing up the ER-3 and just restoring to the ER-5 would actually work.  And then from there I can just continue configuration of the ER-5 for the additional ports.

Thanks!