The billing for your new and larger iCloud+ account will commence from the moment you make the change, and your old plan will cease. You will be reimbursed pro-rata for the amount of the monthly plan you have not used.
Likewise, this happens to any family member whose billing for an existing iCloud+ plan ceases when they accept the invitation to join the family sharing group iCloud+ plan.
Just make sure that your new iCloud+ family group plan which you intend to share is large enough for your needs before inviting the second person. You will need to have an iCloud+ plan in pace which can accommodate the space already in use in the two individual plans.
With regard to the existing iCloud+ plan what is it you want to verify? Are you or your wife paying the account's monthly billing?