Tell ATS.
Not because you have to but because it is good Airmanship.
Stateside, pilots report lots of things in PIREPS and the availability of this information makes the system safer for those that use it.
Things like icing, turbulence, IMC when it is forecast VMC and VMC when it is forecast IMC should be passed on whenever possible because it helps other pilots make important safety related decisions.
I would not like to be the pilot who departed VFR enroute to X but encountered unexpected IMC downroute and used the good old IR to get out of trouble..........did not bother to tell anyone about the unforecast IMC conditions only to find out that an inexperienced PPL who departed VFR 1 hour after me on the same route crashed after a VMC into IMC mistake.
If things are not as expected tell people - it's good Airmanship
Regards,
DFC