Definitely get all that old soil out of there; water movement will be messed up if there are two different kinds of soil with different particles sizes in there. Swish the roots around in a bucket of water, use a tool to work out hardened clumps, swish again. This is also when you'd prune the roots to remove the ones that that are woody, thick, circling the pot, or going straight up or down. Use a spray bottle to keep the roots from drying out and you should be okay.

I pot up a lot of perennials in containers and I never, ever bare root them. IME over many years of doing this, I have never seen the need to be that brutal to actively growing plants and I have also never encountered any problems by NOT bare rooting them. It may be counter to "accepted" container practices but the need to completely remove all existing soil is, I believe, rather overstated :-)

I do use a high quality fast draining mix and when I get around to unpotting the containers down the road - usually when I decide to switch out the plantings and get the current plants in the ground or divide them or whatever - the original soil has pretty much dispersed throughout the mix and the roots will fully fill the container.

It may be just an experiential thing - I've been doing this a long time and have gardened successfully almost exclusively in containers for many years - but I have never encountered drainage problems or developed any root rots by following this practice. For less experienced container gardeners, following the recommended processes may be easier/safer - YMMV.

Alright well since I'm not too experienced I guess I'll semi bare root it like take off part of the soil but not all. Gardengal do you use the 5-1-1 mix for your perennials as well?

I tend to assume that people who are potting up are talking about a tight root ball that contains peaty hardpacked soil, where the water movement in the ball is very different than the movement in the new soil surrounding it, which is what would happen with a compact root ball in 5-1-1 mix. I agree, though, that it certainly isn't as much of an issue if the root system is loose and open enough that its margins will blend with the new soil.

That being said, I recently potted up a rootbound plant that dried out within a day of watering, so until repotting I decided to pot the undisturbed root ball in 5:1:1 mix. According to my dowel, watering left the inside of the root ball bone dry while the 5:1:1 got damp, because water drained before it could get to the roots, so now I immerse the pot in a bowl of water to make sure both the mix and the root ball get wet. A few days ago I accidentally knocked the pot over and while ticking it in again I saw root growth in the root ball and reading out into the 5:1:1 mix, which means it will make it until I repot next month.