There is a standard process for delaying a flight plan, and I think a delay does not trigger a re-validation so should be fairly safe.
Bringing a flight plan to an earlier time cannot be done; one has to cancel it and file a new one, and that definitely triggers the validation process, which can cause a rejection.
Are there any other aspects?
I am surprised that (if) a delay does not trigger a revalidation, because you could delay a flight to a time on which there is a military exercise on the route and the original route is no longer available. How is this handled?
You can file flight plans that do not validate, you are just supposed to not do it and the tools that offer you to file Eurocontrol IFR flight plans may not let you file unless the plan validates.
In fact, you can put “I WANT TO FLY FROM SHOREHAM TO LA ROCHELLE” in the route field of your flight plan and it will go to a manual processing desk. In case of Eurocontrol, they will toss it because it requires more than a small correction, in case of DFS they will most likely do it for you and in case they don’t want to, give you a call 
You can file flight plans that do not validate,
How?
You just get a REJ, surely?
I used to have a contact at Eurocontrol who told me he has a means of forcing a FP into the system no matter what it is, but that is not open to the great unwashed.
No, you can file it, the answer will be “manual processing queue”. Pretty much anything can be filed, surely any kind of nonsense in item 15 (the route).
You could also file via AFTN and address it to all affected ATS units, then the plan is in the system. Eurocontrol provide that distribution for you, you just address it to their two AFTN addresses and it determines the ATS units for you but all they get is a standard flight plan (usually in ADEXP format, not the old ICAO format but it depends on how the receiving unit is setup with Eurocontrol).
Are there any other aspects?
The only thing that I can think of are airway slots (CTOT). By delaying or anticipating a flight plan, they usually get much worse than before.
Otherwise I have not had any problems yet when pulling a flight plan forward (or re-filing the flight for an earlier EOBT). Due to the nature of our business we rarely depart at the scheduled time. About 1/2 of the flights need to be delayed, 1/4 depart earlier and the last quarter leaves on time. Here in Germany (and some other countries as well) calling for start-up 30 minutes before or after the EOBT filed in the flight plan is usually no problem and does not require a new plan, airport slots apart.
there is a military exercise on the route and the original route is no longer available. How is this handled?
tactically, i.e. the affected ATS unit will just route you around the exercise area.
You just get a REJ, surely?
But only after the guy at the manual queue in brussels (or bretigny) pushed the “reject” button. But 99% of the time this is what happens if the plan does not validate (except for POGO flights)
No, you can file it, the answer will be “manual processing queue”. Pretty much anything can be filed, surely any kind of nonsense in item 15 (the route).
Yes but then it isn’t actually filed. It will get REJected after a few seconds so you achieved nothing.
You could also file via AFTN and address it to all affected ATS units, then the plan is in the system. Eurocontrol provide that distribution for you, you just address it to their two AFTN addresses and it determines the ATS units for you but all they get is a standard flight plan (usually in ADEXP format, not the old ICAO format but it depends on how the receiving unit is setup with Eurocontrol).
That’s clever. I am sure London Control hope not too many people discover this 
It would prevent them tossing out flight plans filed for 2400ft, OCAS, etc…
BTW, my experiments show that you can still delay a flight plan until 24h after the original EOBT. I haven’t tried it in real life but the Eurocontrol system keeps the flight plan for 24h and allows delays past the filed EOBT. I don’t know how the ATS units would react because they get the flight plan 2 hours before the EOBT and I wonder if it says there for 24h as well.
Easy to test though and I will do that soon
they get the flight plan 2 hours before the EOBT
The info I got from Eurocontrol is that this time is configured by each country and is generally 10 hours.
I am fairly sure 2hrs is not the UK case, based on what I have seen by phoning up my local airport to check they have a FP I filed.
According to the manual for our Swedish FPC website you can delay your flight plan up to 3 hours after EOBT.
The new departure time must be at least 15 minutes later and up to 20 hours.
Delays between different days (23:00→01:00) is not recommended.
