Bathroom Remodel with PEX conversion

I am DIY'er (or at least trying to be). I am remodeling a bathroom for the 2nd time. I used all copper for my 1st project. I am attempting to use PEX for this 2nd project and I am not sure if that is a good idea.

The new bathroom will have 1 sink, 1 toilet, and a shower/tub so 2 hot lines and 3 cold. There is an existing 1/2 inch copper line supplying the cold and an existing 1/2 inch copper line supplying the hot. This bathroom is on the 2nd floor. Both of those supply lines 'arrive' at the bathroom at an access panel that is accessible from a bedroom closet. My plan, hopefully, is to install 2 new PEX manifolds at that access panel and run the 3 cold PEX lines (1/2 inch each) and 2 hot PEX lines (1/2 inch each) off of those 2 new manifolds.

Here is my problem/question:
All the manifolds I have seen have 3/4 inputs. As I said above, the hot and cold supply lines to the bathroom are 1/2 inch copper. If I convert those supply lines to 3/4 inch and connect them to the new manifolds, will I have water pressure issues?

Thanks for any help!

Yes, that will work.. When we do new construction (all pex) we try our best to pull each fixture a 1/2 line off a 3/4 trunk.. That being said all you have is a 1/2 feed line so it is what it is. BUT 1/2 copper is almost the same inside diameter as 3/4 pex fittings so you really wouldnt be losing much volume as compared to having a 3/4 pex trunk line. make sense? Now this is all supposing that you have adequate supply and pressure already. Also in a 2nd floor bath your going to lose a little pressure (may be noticable, may not) One thing to remember, when you convert the copper to pex you need to be sure you dont make the conversion from 1/2 copper to 1/2 pex and then increase to 3/4 pex. You need to convert the 1/2 copper to 3/4 copper to 3/4 pex.. or some other method of conversion without using 1/2 pex at that point. Most DIY folks would probably use a 1/2 x 3/4 sharkbite to easily do this.. I dont sharkbite, so I wont recommend that you do.... but lots of plumbers do. in the end its up to you

Converting to 3/4" at the manifold then back to 1/2" won't change the pressure. You're still limited by the size of the inlet pipe, and that might be an issue trying to feed two baths with a 1/2" line. It's the total flow available that's an issue.

Thanks guys. The supply lines are only feeding the 1 bathroom ( 1 sink, 1 toilet, 1 shower/tub). From what LiquidPlumber says, it sounds like converting the 1/2 copper to 3/4 copper and then connecting to the 3/4 PEX manifold will be just fine. The manifold will run 1/2 PEX lines to the fixtures. I did NOT have any pressure problems with the original/old fixtures so it seems safe to assume that setup will work just fine. That is very good news. Using PEX will be new to me but, if all goes well with this 1/2 to 3/4 conversion, then I will continue to use it. Thanks!

Why go to the cost of a manifold? Since you only have a 1/2" feed just "stack" a couple of tees on top of each other and run to the faucets.

Sorry, I am not sure what 'stack a couple of tees on top of each other' means? And, 'yes' those manifolds are pricey!