SkyDog75

There are adapters to go the other way, fitting threaded bottom brackets to BB30/PF30 frames, but you can't physically fit a BB30/PF30 bottom bracket and crankset to an English-threaded bottom bracket shell. They're both 68 mm wide, but BB30/PF30 requires larger bearings than you can fit into a threaded bottom bracket shell. And you can't mount bearings outboard, or the shell plus bearings would be too wide for the spindle of a BB30 crankset.

There are LOTS of bottom bracket and crankset choices for English-threaded bottom bracket shells, though. It's been the most common standard for decades, only losing market share to other bottom bracket types in the last few years.

If the problem with chain tension started happening when you replaced the cassette, it's possible you installed a cassette that exceeds the take-up capacity of your rear derailleur. You need your chain to be a certain length to reach your big chainring + big cog combo. When you shift to the small+small combo, there's going to be slack in the chain, which is taken up by the derailleur cage swinging back. The greater the gearing differential between the big+big combo and small+small combo, the bigger the difference in required chain length, the more slack there'll be in the small+small combo, and the longer the derailleur's cage needs to be.

If that's the case, the easiest fix is to swap your rear derailleur for one with a longer cage. Without knowing specifically what derailleur you're using, I can't give you more specific advice. The other easy fix is to replace your cassette with one that doesn't exceed your derailleur's wrap capacity. Replacing your crankset or chainrings could fix your "chain tension problem" by reducing the difference between your big+big and small+small combos, but there are a lot more variables and potentially more expense involved: crankset, chainrings, and/or shifters, even. And you'll possibly lose some gear range, too.

If you're still interested in swapping chinrings, the answer to your question about whether it's possible is "possibly". It depends on what kind of crankset you currently have. Most better cranksets have chainrings that can be unbolted. Lower-end ones have non-replaceable chainrings. If your chainrings are removable, you can certainly remove one and/or replace them. If you go to a double, whether by going to a new crankset or removing a ring, you may have front shifter & derailleur considerations to take into account.

Oh, and going back to chain length and derailleur capacity for a moment... You mentioned the derailleur being pulled so far forward. Did you swap your chain for a longer one when you installed a bigger cassette? Your current one might be shorter than ideal.