My best friend, sailor mentor and all around good guy did exactly what you are proposing, moved from a 22 ft Catalina to the 30 with the tall rig
. I spent many days and evenings with him on both boats. He's not on sailnet so I'll proxy for him.
If you are used to getting on and going, there will be a difference. Maintenance time being one, he spends about 1 hour on maint time for every 2 sailed on his 1986.
Speaking of maintenance you will need to learn the boats systems. The Catalina's lay out and equipment options are pretty simple and not a great hurdle, my buddy is dsylectic so I spent the time reading the manuals an showing him hands on. I like most of what they have done, although not up to todays standards older catalinas will serve.
His boat had serious blisters, check yours thoroughly and know what you are getting into if you decide to take on the project of fixing it.
He had to replace / re-bedd every portlight on his boat; he went for quick rather than good and put the same units back in. If you do this on your boat I'd recommend some more opening portals on the sides. New Found Metals makes good alround coastal ports for good prices.
The only other problems he's had - a water separating fuel
filter was not standard on his, I plumbed it in for him. He has the universal 25; which is a marinized tractor diesel, easy to find parts at non-boat prices which is always good.
His battery/12v system was a mess of previous owner wiring and flat batteries. We wired in a complete replacement of it from battery to beltdowns and the 110v system over 4 weekends - about 400.00 bucks, not including new charger and house batteries. MkIII being newer you might not face this.
His only experience on boats bigger than 22 were a couple times at the helm of my Hunter 31 - he had no problem transitioning to the Catalina 30 - even backing it into his slip became simple once he practiced it a few times.