If you ONLY had the 17's I think B&O would recommend you use them as rears as they would help fill the space around you better and bring more bass to the back of the room.  However the 2500's aren't the most modern or powerful speakers so using them as front L/R might overwhelm them. At the very least they will not sound anywhere as good as having the 17's in the front. The 2500 would be good as rear surround speakers as they would really just be filling the void of sound behind you which are never really all that loud or demanding.  Paired with the 17's at the front it should all sound very impressive. Since the theatre won't be using the L/R built in speakers you can re-assign them to act as Centre (if that sounds too wide is up to your own taste) or you can set them to Sub Centre speakers for an extra kick.

Sounds good.  I shall try it out and update on how it goes.  I will be getting the powerlink cables for the 2500 this Friday.  I shall put the 2500 as  rear surrounds and keep the 17 in the front.  Thank you for your advice.

I finally got the cables today and installed the beolab 2500 as rear speakers and the 17s in the front.  It sounds great.  But a note, the app did not have a listing for beolab 2500.  I used the beolab 4000 from the listing instead.  I thought they would have included all speakers from the late 80s onwards.  Would you recommend any other speakers that would match the beolab 2500 for the configuration?  Thanks