If you change your plan in the middle of the subscription period and you have not yet used up your current reference allowance, the remaining reference allowance will extend through the first subscription period (month/year) of your new subscription plan.
When your new plan renews, the reference allowance will reset to the new starting total.
For example, Eddy Fix has a Monthly Basic subscription, which allows Eddy to Edifix up to 250 references per month. In the middle of the month, Eddy can still Edifix 50 references, but he decides to upgrade to the Monthly Plus subscription, which has a reference allowance of 500 references per month. When Eddy changes his subscription, he gets a one-month grace period to use the 50 references from his old allowance. In other words, for the first month of Eddy's new Monthly Plus plan, he can Edifix up to 550 references. At the end of the month, his allowance resets back to 500 references.