Jetpack Search: Convert monthly plan into annual

#### Steps to reproduce the issue

1. Purchase a monthly plan
2. Purchase an annual plan
3. Notice you'll have two active subscriptions for Jetpack Search

#### What I expected
For the monthly plan to be converted into an annual plan

#### What happened instead
The user ends up having two subscriptions for the same service

#user-report

25814956-hc

Start by reproducing the two purchase steps described in the issue and trace the Jetpack Search subscription-handling entry point. Done means converting an existing monthly plan to annual without leaving two active subscriptions for the same service; the payload names no files or tests, so the relevant coverage must be located.

Written by the indexing model from the issue text.