Hello All. I'm thinking of upgrading to Platinum from 8.5 and was wondering if the 8.5 recorded files will work in Platinum? Also I use Percussion Strip a lot and again was wondering if that is included in Platinum? I also do a lot with midi synths and wasnt sure if it comes pre-installed with any? Thank you.

Hi, and welcome to this forum. Yes the projects from 8.5 will load in SONAR Platinum. You can even leave 8.5 on your machine to get access to the plugins that are not included in Platinum. 
All the best.

FYI, this is a list of the included plugins with Platinum:
 
 
Instruments
Addictive Drums 2
Cakewalk Sound Center
Session Drummer 3
Square I
Roland Groove Synth
Cakewalk TTS-1
DropZone
SFZ Player
Studio Instruments Drums
Studio Instruments Bass
Studio Instruments Strings
Studio Instruments Electric Piano
Dimension Pro
Rapture Full
Cyclone
PSYN II
RXP
Z3TA+ Classic
Lounge Lizard SONAR
Strum Session 2
TruePianos Amber
Rapture Session
Ultra Analog Session 2
 Anderton Collection
Acoustic Piezo CA-X Amp Sim
CA Acoustic Guitar Presets
CA Amps
CA VoxTools
FX Chains
Kickmaster FX Chain
Monitorizer
Pedalboard FX Chains
Phasor Constructor
Quick FX
SizzleBus
 Essential Suite
Sonitus Compressor
Sonitus Delay
Sonitus Equalizer
Sonitus Gate
Sonitus Modulator
Sonitus Multiband
Sonitus Phase
Sonitus Reverb
Sonitus Surround
Sonitus Surround Compressor
Sonitus Wahwah
 Classic Creative Suite
AliasFactor
Chorus/Flanger
Classic Phaser
Compressor/Gate
HF Exciter
Modfilter
Para-Q
Tempo Delay
 Studio Mixing Suite
Analog Chorus CH2S
Analog Phaser APH2S
BlueVerb DRV-2080
Brickwall BW-2S
Compressor CP-2S
Compressor FA770
DeEsser DS-25
Equalizer BQ2S
Equalizer BX2S
Equalizer GEQ12
Equalizer PEQ2B
Equalizer PEQ5B
Equalizer PEQ322
Gate Expander GX622
Limiter LM2S
Oilcan Echo TLE2S
Stereo Imager ST2S
Tempo Delay 3D
Valve Driver ADR2S
 Engineering Suite
Boost 11
Channel Tools
LP EQ
LP MB Multiband Compressor
 Boutique Suite
TH3 Cakewalk Edition
Analog TrackBox
BREVERB SONAR
PX-64 Percussion Strip
TL-64 Tube Leveler
TS-64 Transient Shaper
VX-64 Vocal Strip
Tone2 Bifilter2
 ProChannel
Bark of the Dog
BREVERB SONAR
Console Emulator
Panipulator
PC4K S-Type Bus Compressor
PC76 U-Type Compressor
QuadCurve EQ
REmatrix Solo
Softube Saturation Knob
StyleDials
Tape Emulator
Tube Saturation

You'll have no problems migrating 8.5 projects to SPlat, as long as you leave 8.5 installed. 
 
You will pick up some new instruments (Addictive Drums, Strum Acoustic, Lounge Lizard, full Rapture).

Yup - I concur on the earlier posts.  I just wanted to reiterate that it is perfectly OK to have multiple versions of Sonar on your computer - and many folks have 2-3 or even 4 releases of Sonar installed at the same time.  (I have X3 and Platinum on mine, for example).
 
And, even though one or more plugins from 8.5.3 were left out of newer releases of Sonar, your 8.5.3 plugins can continue to be used in newer Sonar releases.
 
I believe the Cakewalk Plugin Manager and possibly some of the utility stuff might be shared, so that if you install Platinum the Sonar 8.5.3 will use the newer Platinum version of the Plugin Manager, but that didn't hurt anything - I am just letting you know that some of that stuff is shared among multiple versions of Sonar, so you won't be surprised or freaked out to discover that. :)
 
It is also important to note that if you create or save a project in Platinum, it may not be able to be opened from within 8.5.3 anymore, but your 8.5.3 projects should open up in Platinum.  Some folks at some point a while back (might have been X3 or even X2) reported that some of their Cakewalk Bundles had trouble opening up in later versions of Sonar.  But regular projects should be OK.
 
Bob Bone