Hi @RumorConsumer
I believe this may not be possible.
For every MX Model there is a different kind of License. MX65 License cannot be used for MX84.
You are right for warm spare one should have similar unit of hardware. However you do not need 2 Licenses for HA as the mode is Active - Standby. Hence 1 License will do. Having said that in your case this may not be of any help as the Hardware's are of 2 different models.
Regarding configuration backup I do not see much options available. The thread you are referring to shares appropriate information while replacing MXes.
I believe @PhilipDAth had a script to download the configuration. I am not very sure though.
Regards,
Ajit
AjitsNW@gmail.com
www.ajit.network