Definitely. I was really looking at building my current project up into a 6.5 Grendel, but reasons that I will share made me change gears just a bit.
First off, yes. You can use your existing lower, which is a great savings and one of the truly great things about the AR platform. God I love them... LOL
However, you will need a bifferent bolt, different bolt head and, obviously, a different chambered barrel. It's far easier to get a whole different upper and just swap them back and forth, but if cash is super tight, you can break down your existing upper and put those parts in to transform it into a Grendel. More work, and obviously with the removing of the barrel, don't expect your zero to be anywhere near where it was the last time you had your last caliber on that upper... LOL
My 'smith and I really discussed the 6.5 Grendel, and I was really leaning that way, but after some ballistic comparisons, it came down to the 6.5 Creedmoor by more than a nose to be honest. I guess the Grendel cartridge has a large primer, where the Creedmoor has a small primer. One of the differences is that the pressure builds SLIGHTLY
slower with the smaller primer, so you get a more even ignition. More even ignition leads to more consistency between fired rounds, leads to the rounds heading down range the same way, leads to tighter groups if I do my part.
In any event, the Creedmoor has some great ballistics and the guru's from benchrest land have already done ALL the work for me, where the Grendel is pretty much brand new and there are only a couple of brands ( Hornady I think, Black Hills and some stuff from Les Baer ) that are making "factory ammo" for it. I saw a post where one guy bought like 500 rounds for like $600 from Les Baer, but his rationale was that it came with Lapua brass, so he could reload it. I don't know about you, but that is pretty pricey ammo in my book.
All in all, everything I initially looked at with the Grendel was positive, but once I got to discussing it with Brett, my gunsmith, we looked at the performance and advantages of the Creedmoor and it seemed to make better sense for me at this time.
Way too much information on the Grendel here.
If you end up going Grendel, please be sure to post the results and your build specs. I would love to see the project!