There are separate numbers.
The upgrade/modification period is 14 days from first use. If your first day of use of a ticket is the 1st of a month, once you reach the 15th of that month you can not do any modifications or upgrades to that ticket.
The ticket has various "entitlements". The base ticket has a certain number of days of entries to the four major parks. Having WPF&M on the ticket entitles you to a certain number of entries in the minor parks.
If you do not purchase the No Expire option all of you entitlements expire after the 14th day of first use.
If you purchase the No Expire option any unused entitlements will not expire. This does not override the upgrade/modification limit.
Note that even if you have used up all of your entitlements and it is within the 14 day period you can still upgrade or modify your ticket. For example, if you had a five day hopper and had used up all five days and suddenly discovered that due to a really severe snowstorm at home you (unfortunately) had to stay two extra days at WDW you could spend the $4.00 per ticket to upgrade to a seven day hoppers instead of buying two day tickets for $125 each.