My experiences...

Have three cards, often get lent out to visitors. Topped up some, both, all variously until faced the registration request. Registered it, subsequently registered all the other cards - don't want friends having to deal with this.

One card expired (5 years), got a new card, balance transferred, don't think I was charged a fee, original card was no fee/deposit. No registration issues. Tried to top-up later, must register. I use my Thai DL for registration.

Second card expired, got a new card, balance transfer, no fee, but had to register it, Fine. DL out. Tried to top-up this second card later, nope, have to register, again. OK, DL out.

Third, oldest card now, no issues topping up, yet.

When I'm behind a Thai person in line to top-up it seems like the BTS agent requests the Thai ID card for a top-up. Thai ID cards have a chip, and the BTS agent has a reader, pretty quick process for them.

Not sure about the variations in experience?

On a side note: newer train sets (not sure if CNR or Siemens) operating on the Suk. line, spiffy, two of eight seat sets removed to increase capacity. LED screens for route/station info. These could have been on the Silom line, which I haven't ridden in a while.