Straightening a bend copper tubing of an air conditioner filled with refrigerant

From Pex Information:

• Flexible PEX tube is manufactured by extrusion, and shipped and stored on spools, where rigid plastic or metal piping must be cut to some practical length for shipping and storage. This leads to several advantages, including lower shipping and handling costs due to decreased weight and improved storage options.
• PEX plumbing installations require fewer fittings than rigid piping. The flexible tubing can turn 90 degree corners without the need for elbow fittings, and PEX tubing unrolled from spools can be installed in long runs without the need for coupling fittings.
• Attaching PEX tube to fittings does not require soldering, and so eliminates the health hazards involved with lead-based solder and acid fluxes; PEX is also safer to install since a torch is not needed to make connections..
• PEX resists the scale build-up common with copper pipe, and does not pit or corrode when exposed to acidic water.
• PEX is much more resistant to freeze-breakage than copper or rigid plastic pipe.
• PEX tubing does not transfer heat as readily as copper, and so conserves energy.
• Water flows more quietly through PEX tube, and the characteristic "water hammer" noise of copper pipe systems is virtually eliminated.
• PEX plumbing installations cost less because:
  • PEX is less expensive than copper pipe.
  • Less time is spent running pipe and installing fittings than with rigid pipe systems.
  • Installing fewer fittings reduces the chances for expensive callbacks.

If you are only going to be doing a little bit of work, check with your local rental stores, you may be able to rent the tool for a decent price.

Yes you can do that (it's the standard way of converting to/from copper), and it shouldn't cause any troubles (read Why would you use copper over CPVC or PEX? and Does Pex tubing have any advantages over copper? for some comparisons).

Of note: you can get the male adapters that fit on the outside of copper pipe, and also that fit on the inside of copper fittings (like a street elbow). Can be handy sometimes if you're working in a tight space, or were going to immediately make a turn anyways (since you're soldering the fitting anyways, it's barely any extra work to do an elbow+adapter, and saves a PEX elbow and couple of crimp rings).