Well, you know Ryzen is a completely different platform, so you can't avoid changing the motherboard there.
Sadly, Intel are greedy pricks and deliberately force it's customers to change mobos every other generation.
Even though your 6600k, a 7700k, 8700k, and 9600k all fit Socket LGA 1151, the pricks at Intel separated them on the bios level - no way around it:
6600k - compatible with 100 and 200 series Intel mobos
7700k - 100 series(requires bios update) and 200 series
8700k - compatible with 300 series Intel boards only
9600k - requires bios update on all 300 series, except B365 and Z390
Besides compatibility, the other thing to consider about a motherboard is features.
[This is just a generalization, there are exceptions.]
1)M-ITX: the small form factor, the barebones option, the cheapest(usually), limited variety, fits pretty much all cases
-You just want to slap in a cpu, a couple sticks of ram, 1 - 4 storage drives, a gpu, and be on your way.
2)ATX: the standard, a few extra features(bluetooth, wi-fi, more PCIE slots, etc), the middle of the road, when the small form factor offers too little, the most variety, fits most cases
-You need a little more than just the barebones. Perhaps you want to add a sound card? A wi-fi, or network adapter? A fan hub(add in card)?
Up to 6 or 8 storage drives? M.2? Multi-gpu - SLI/XF?
3)E-ATX: the Big Daddy, loaded with features, the Bank Slammer, when you want it ALL, limited variety(it already has everything!), limited case compatibility(usually restricted to the largest cases).
-You want ALL of the features, have 10+ storage drives, and throw value out the window to get it.
It's ultimately up to you to choose to upgrade the motherboard. If not, the 7700k is as far as you can go, and as I said before, those things are still not cheap, even in the 2nd hand market.