Is set.seed consistent over different versions of R (and Ubuntu)?

I am currently running R version 3.1.0 (on Ubuntu 12.04 LTS) and as both my R version and my operating system is getting rather old, I plan on updating both. However, I have a lot of simulations that rely on set.seed() and I would like them to still give me the same random numbers after updating both R and my operating system.

So my question is three-fold.

1. Can I update R without changing which numbers are generated from each seed?
2. Can I do the same for my operating system?
3. If no to either 1) or 2), is there a way to change the seeds in my code in such a way that they are consistent with the olds seeds?

Cross-OS consistency: yes

If you installed R on two different operating systems without manually changing defaults or the RProfile, you should get the same results when using set.seed().

Consistency over versions of R: not necessarily

It used to be the case that set.seed() would give the same results across R versions, but that's no longer generally true thanks to a little-announced update in R 3.6.0. So you can get cross version consistency comparing results before R 3.6.0, but if you compare a post-3.6.0 use of set.seed() to a pre-3.6.0 use of set.seed(), you will get different results.

You can see that in the examples below:

R 3.2.0

> set.seed(1999)
> sample(LETTERS, 3)
[1] "T" "N" "L"

R 3.5.3

> set.seed(1999)
> sample(LETTERS, 3)
[1] "T" "N" "L"

R 3.6.0

set.seed(1999)
sample(LETTERS, 3)
[1] "D" "Z" "R"

The reason for the inconsistency is that in R 3.6.0, the default kind of under-the-hood random-number generator was changed. Now, in order to get the results from set.seed() to match, you have to first call the function RNGkind(sample.kind = "Rounding").

R 3.6.0

> RNGkind(sample.kind = "Rounding")
Warning message:
In RNGkind(sample.kind = "Rounding") : non-uniform 'Rounding' sampler used
> set.seed(1999)
> sample(Letters, 3)
[1] "T" "N" "L"