Advice on replacing internal bearing BB with external

I'm putting together a parts list for converting a road bike to use a compact crankset. While I'm at it I also figure to update the integrated spindle cartridge English BB (BC-1.37"x24Tx68mm) by replacing it with a thru-spindle external bearing BB. But I'm afraid I haven't paid attention to all the new BB formats created in the past decade or so, so I would appreciate some tutoring on "converting " an English BB shell to a BB with external bearings, chiefly which companies make a BB that will work with my BB shell, and which among those have the best reputations.
I'm assuming you're using a hollowtech 2 crank? Standard Shimano BBs are fine. If you want to go fancy you can get Hope, Chris King etc. so you're not replacing the whole thing each time and just replacing the bearings.
The crank is Isis so the plan always was to change the BB along with the crank so my crank options wouldn't be limited, and the current BB is long in the tooth anyway.

What I'm lost in the weeds about how the 'conversion' is done, whether there are external bearing BBs that will thread right in to the BSA shell or whether there's an adapter involved.
Sorry, I meant assuming you're changing to a HT2 crank.

There's no adapter involved in terms of them fitting your BB, external bearing BBs are for the same width shell as an isis/square taper/annoying Sugino taper BB, they thread straight on and the majority are BSA threaded. If you're running a HT2 crank then you just need a set with a 24mm ID. Shimano do several levels that all work fine, or you can go for the bling with Phil Wood, Hope, Wheels Manufacturing, Chris King etc. The benefit of those BBs is you're only changing the bearings when needed, the cups stay in the frame, so it's less waste and less wear and tear on the threads (although that second point really doesn't apply much if you're installing things properly). The Shimano Ultegra and DA work just as well as those fancy ones and I think they're a little lighter as the cup and bearings are smaller. This has an impact on longevity, especially if you're riding in poor conditions. It's the Same for Campag, but you need their cups (they make them in BSA/English threading).

If you're going to look at different cranks (Rotor, FSA etc.) then it's the axle diameter and ID you need to pay attention to, because many brands will use different standards.

In terms of adapters, these are usually only needed if you're going to run something like a Hope of Chris King BB with a GXP crank. The axle on a GXP crank is taper from 24 to 22mm, so rather than make loads of specific cups, they just made an adapter to shim the axle. Personally I'd stay away from doing this (I run a GXP crank on a fixed gear I have and I've found they can be noisy).
Thank you for that, King Bonnen. That clears up much of my confusion.

I built this bike the same year that Tyler Hamilton broke his collar bone in the TdF prologue and his team switched him to a compact crankset so he could continue the race and do his climbing without having to get out of the saddle. It's got 10-sp Record derailleurs but I had FSA make a bespoke Isis crankset for it with a 130 (Shimano) BCD so I could run a one fewer teeth on the small chainring than had I used a 135 (Campy) BCD. So my 51x38 setup was part-way to compact crankset but without the "stigma" attached to old man gearing. 18 years on and I'm thinking that the time for that "old man" gearing might have come, so that's why I'm dust binning a still serviceable BB along with the crankset.
I know very little about this subject. There are dozens of different subject videos on YouTube about bottom brackets, design, thing you get right a little research..and spend enough money for quality and proven designs and manufacturers..
Don't buy anything cheap..