UBB.threads PHP Forum Software Community Forums UBB.threads 7 Bug Reports Insecure password warning, Firefox 52

I see that my hosting service (DreamHost) offers some help with the free 'Let's Encrypt' certificate.

Questions for the experienced UBB admins:

If I install the certificate, will visitors' browsers automatically find the https URL, or will there be a period of questions and problems while everyone has to figure out the change?

Is it reasonable to encrypt the sign-in and leave the rest un-encrypted, so that the main forums remain under the http://... URL while the sign-in screen uses the secure https://..., or is that a pain to set up?

Thanks!