Pacemaker upgrade to a CRT

[Nomis21]

53 pacemaker dependant after pace and ablate, total heart block.Never really thought what would happen if my 2 lead pacemaker failed? I know this is very very unlikely but just curious, can anyone enlighten me?

Also I'm planned to have a upgraded 3 lead CRT pacemaker due to HF (have appointment next week with cardiologist to discuss)

Guessing it's a complete swap of pacemaker models, how would they do this if this is the case If I have total heart block?