Just wanna check. if i'm currently on the $8 plan and i use up my 2GB int roaming. If i upgrade my plan to the $18 which should take effect immediately, does that mean i will have 10GB more int roaming?
Like what Habrosus mentioned, switching to $18 plan is immediate, whatever you have previously is history.
So from the point of switch, for the next 30 days, you will have 10GB international roaming, period.
The word more don't really apply here.
OTOH, unless you really expect to just barely exceed 8GB international data, I would suggest switching to the $11.80 plan for 8GB international data instead.
If you are already expecting to exceed 8GB, another 2GB is probably not going to be enough. At the time you are about to exceed the 8GB, you can probably make a better call, on what is your best course of action.
BTW, if you travel with another person that uses very little data, another option is to swap sim card when starting to run low on data.
Also consider a spare sim or port to another telco temporary if that telco has something that suits the country in question better. Eg, eight has 2GB for Japan, Zym/Zero1/Vivifi has $15 plans with 15GB for Japan. If you expect to need >10GB, port over might make sense.
BTW, if you are renewing multiple times within a trip, you might to stop and review what you are using data for.
I barely hit 3GB in a typical 2 weeks trip, that is when using the phone heavily for maps, translation and search.