Sag Front Lenses

[TVR Cambs]

I've read somewhere that removing Sag front headlight lenses to then reseal them is a tricky job which can lead to them cracking. Has anyone done this job themselves? If the lenses crack then is it easy to get hold of replacements? I'm assuming that if the sealant has come away then the seal will let in water and the lenses will mist in rain? Lots of questions sorry, thanks for your advice