Ditto....
Your music should start to have a cohesiveness about it as time goes on. The songs you record should start to sound like they were all recorded in the same studio at around the same time. This comes from refining your work flow into repeatable patterns, and using the same plugs and settings (pretty much) once they are "working for you".
So when you want to "master" a project consisting of several songs..... popping them into one new project.... I would drop each song into a different track..... set up the lightest mastering in the master bus and delicately work on them in that manner.If you have consistency in the tracks, this is all that is needed,,,,, perhaps even unnecessary.
If they are worlds apart sonically, it would be necessary to go back to the original project. Unless you wanted to fix an issue in the mix, it's not necessary to mess with the mix and the levels. Simply remove the mastering plugs, EQ, reverb, compression, etc,,, and do a SAVE AS so you still have your original project should you ever want to get back to it.
Make notes on what plugs you use, where you use them, the presets or settings used, and replicate that in each tune in the project. It might end up being a lot of work but that's what would be needed to get them all sounding like that are the same band, in the same studio, at the same time.
Here's an example from my own archives:
Listen to the fiddle. In the first example, the mastering plug had the fiddle sounding like it was under a blanket. In the second example, I had the same exact fiddle track, but I deleted the plugs all the way around and started fresh. It was a remake of the song but the fiddle track was brought in to the new project and the mastering of the project was done from scratch. The second example is so much more open sounding..... just an example.
example 1
http://soundclick.com/share.cfm?id=7520478example 2
http://soundclick.com/share.cfm?id=11962059 Don't be afraid to start over to get the tracks sounding better and working together as a unit especially if you're putting a collection together.