I replaced my left side solenoid and cam guide with the engine in the car. As others have said, this is not very easy, but for a 20 year old car spending $2.5k - 3k to have it done doesn't make sense. It did work out for me and the engine runs like new now, but I would not want to try it again.
Here's that thread:
http://986forum.com/forums/general-discussions/60995-vario-cam-adjuster-plunger-question-re-p1324-code.html
My biggest hurdle was reassembling the cam shaft under the cam cover, which contains the bearing caps as part of the casting. From what I could see with the placement tools that I bought from Pelican to hold the cam in place and hold TDC timing, it seemed nearly an impossible feat, but I took my time and figured a way to use the car's frame and push against the cam cover to seat the camshaft and get the bolts into place.
I posted about this last year with photos, but in a nutshell, used a turnbuckle and ground grooves in the ends of them so they could be used to push the cam cover against the block. During the entire process that took me a couple of weeks head scratching and looking a various how-to's, I kept my hopes up that I wouldn't be parting out my car on ebay to recover what little I could.
Others more skilled and experienced than me might have methods of dropping the engine partly to get better access to the cam covers. I toyed with that, but got lucky with my home made pressing turnbuckle and dodged an expensive bullet.
I'm assuming your codes are aiming at a bad solenoid, though you might find that the error code is coming from the plastic cam runners that wear out. In my case, it was the latter, but I bought a new $200 solenoid as insurance.
If nothing else, if you try and fail, you can pull the engine and fix the solenoid(s) and a host of other parts, like the AOS and clutch while you have it dropped from the car. It's just not worth $3k to have a shop do this in my opinion.