Hi @Sanjmeh!
I understand your concern — it's no fun to get stuck in dependency hell, and it can really make you leery of upgrading anything ever again.
I'm not sure I have a clear picture of your specific setup, but a couple of thoughts:
I don't know exactly what was wrong in the other linked discussion, but as far as I can tell the root of the problem had to do with system dependencies external to R (a C++ library), possibly related to the operating system version being a few years old. Unless you share the exact same system particulars as that person, I'm not sure it's a representative example of what your experience upgrading to R 3.6 will be like.
R 3.5 was a somewhat unusual case, since there were much bigger internal changes than are typical for that level of R release. However, it has long been the case that packages need to be updated — which amounts to re-installation — for every new minor R release (R's versioning system is major.minor.patch, but you may see people refer to the annual "minor" releases as "major" since the actual "major" releases happen extremely infrequently).
Some resources for advice on maintaining R:
If you're dealing with anything more complex than a single-user setup and/or you're concerned about reproducibility of analyses or other data products, I really like the framework for thinking about your options outlined in this webinar:
The related website:
In the interest of knowing what you're getting yourself into before you jump, I found this rundown of notable changes in R 3.6 helpful: