When you purchase a suite, you cannot normally upgrade individual parts of it, you have to upgrade the entire suite. You would need special permission/support from Adobe to be able to upgrade just the one application.
If you have software installed you can continue to use that software even if you upgraded the entire suite.
I am considering the upgrade, but as a semi-retired freelancer, it is an expensive proposition. I have client accounts that require Illustrator and InDesign, but really don't need a Photoshop upgrade. My InDesign client receives his product as an interactive pdf. The Illustrator clients need vector art for flexo. Not sure my volume with either can justify the full Suite upgrade.
Do you have any advice?