You can upgrade your in5 plan at anytime. Most upgrades can be done from your online account by clicking the Edit (pencil) icon next to your plan name, or by visiting the upgrade page directly. You can also restart a previously canceled plan from the upgrade page.
Upgrades are pro-rated, so you get credit for any unused time on your old plan.
If you decide to switch from monthly to annual on the same plan (e.g., Gold Monthly to Gold Annual) that change is also pro-rated to give you credit for unused time on your old.