tenant changing HMO room - new tenancy?

Tenancy Types and Management

An existing HMO tenant - current RLA agreement and RLA guarantor agreement- wants to move to a newly vacated room, about to be relet, in the same HMO as he feels it's quieter; back of building rather than road; all terms the same. He's a reasonable tenant, though lost job and now on LHA. I have no real objection.
Do you think it would be sufficient to sign & witness an alteration to the existing agreement, or would the change be fundamental and need a new agreement? The paperwork would be a minor hassle but the guarantor may not be so easy to re-sign up. If the existing agreement was just altered, would it be enough, or advisable, to advise the guarantor? The sums aren't large, only £60 pw, but I guess guarantors can cut up rough and 'go legal' if pressed for rent or breach costs.
Anything long-drawn-out just stops me reletting whichever room turns out to be vacant. Your words of wisdom would be appreciated!

Nigel Hall