Will Platinun Open X3 Producer Projects?

yes and the other way around.

Yes. Things that don't exist in one version that do exist in the other will obviously fail. Like features only in Platinum will not be useful in X3. But if you create a project and set a compressor in Platinum and then save, close, open in X3... the setting will persist.
 
Next, alter the compressor settings in X3, save, close, open in Platinum and the setting change will persist.

I've had no problems (so far) in the conversion, my projects have opened without any issues at all.  Also my fader port settings and connection were also carried over which is a big plus! It was a headache getting that thing to work with Sonar X3.

X3EP projects will open but you will need to do some adjustments.
My first X3EP project opened in platinum required setting up the audio interface.
Platinum opened with MME driver mode so it did not see my audio interface.
I have 3 display monitors and the Sonar windows did not open like in X3EP.
When I re-arranged the window layout and re-opened the project, Platinum reverted to the messed up layout.
I fiddled around until I could get the layout to persist.
I had to to lock the current screenset which I never had to do in X3EP.
This still needs more investigation.
I had to scan VSTs of course.
 
On the upside, all my X3E plugs used in the project were there, all the pro-channel effects were there, and both these retained their X3E settings. All the channel strip settings were retained.
Melodyne Editor was there and functional (I have builds 254).
AD-1 was functional. AD-2 was functional after I installed it.
Note that I DID NOT uninstall X3E.
 
As far as I can tell Platinum sounds exactly like X3E. This is with FW-1884 (firewire) and EMU 0404 (USB).
The drivers for both these worked as in X3E. Settings were 48Khz/24 with 1024 buffer(yt normal mix).
I have NOT got to recording yet.
 
The test project described, was composed of..
7 Dim-pro's (pro-ch EQs on all)
1- TTS1 synth
1 AD-1 drums (separate kick and snare with comp and EQ, all the rest on AD-1 master bus)
1 Session drummer
1 Session-bass
5- guitar audio trks (pro-ch EQs on all)
5 sub buses - Guitars, Drums, Brass, Woods, Keys some with pro-ch comps and EQ.
The master bus  has BozLabs Mongoose, PC 4K bus compressor, Concrete Limiter.
There is a Master send to an SPDIF main (I use this for the EMU-0404 to slave via Toslink from Tascam).
 
The test project was originally done at another location and moved to my studio.
It taxes my poor old Core-Duo to about 50% CPU utilization and 4 GB of RAM usage.

Kalle Rantaaho
If you're "in the middle of something" shouldn't you just install Platinum  and continue working with X3 and get aquainted with Platinum little by little as you go. I assume it's possible still, as it's always been, to have several versions of Sonar on your computer.

I think that's been great advice on other upgrades, but if you know X3, you'll be really comfortable in Platinum.  It's basically the same with not a lot of work flow changes, that should really affect you switching programs, mid project.  However, don't delete X3 and finish them up in X3, to eliminate ANY possiblitles, of course with a computer hard drive image done beforehand....or continue on with Platinum and enjoy some of the new features like ReMatrix, vocal align, etc...

Kalle Rantaaho
I assume it's possible still, as it's always been, to have several versions of Sonar on your computer.

Absolutely!

There is another post where the OP states Platinum sounds 50% better.