Current LICENSE file in darcs repo contains GPLv2. Would be nice to see
it updated to more recent version: GPLv3+

I'm not sure we could easily do that - would need to contact a 
substantial number of past contributors.

Could you summarise the advantages/disadvantages of changing?

It's already done by others:
https://tldrlegal.com/license/gnu-general-public-license-v3-%28gpl-3%29
and https://www.gnu.org/licenses/rms-why-gplv3.html

Also you should upgrade to GPLv3+ which means "GPLv3 or any later
version" - that's how most open source projects use it to avoid
precisely your problem: it allows to bump license version number without
hassle of contacting contributors.

It's better to that earlier than later - the problem will only get worse
over time and you might be stuck in the same mess with outdated license
as GNU/Linux kernel for example.

god writes:

 > It's better to that earlier than later - the problem will only get worse
 > over time and you might be stuck in the same mess with outdated license
 > as GNU/Linux kernel for example.

Last I heard neither Linus nor David Roundy (who AFAIK still holds
copyright on a lot of Darcs) thinks that GPLv2 is "outdated".

In the case of the kernel, the "mess" is simply the mess that any kind
of fanaticism produces, since either you're in kernel space and you
can either be GPLv2 or use Linus's special exception for modules that
use only public APIs, or you're not in kernel space and you can do
what you want -- there's no legal mess in practice for the kernel
(except for people who want to import GPLv3 code into the kernel, I
guess, but how many people want to do that?  Emacs in the kernel, what
a great idea!)

For Darcs the situation is a bit different, and potentially messy.  It
depends on how much GPLv3 software is becoming part of the GHC ecology
that Darcs links to.