Hi,
If there's a SKY engineer coming to install the new box, ask him if he will do you a bit extra and install a cable run so you can put the box in another bedroom and use it as a freesat box.
IF you do not subscribe to multiroom, you will not be able to use the recording facilities of the older box but it will still serve you well as a freesat box - you could also pay £10 for the multiroom and it will probably be installed F.O.C. and you'll have a second recorder in the house...
The offer of paying £5/month for 8 months then £10/month is for what exactly? The HD subscription? If it is, yes it is a good deal.
Cheers,
Cofnchtr.