Keep both plans and the effective date
Suppose the old June plan is $30 for the month and the new plan is $48. The confirmation says the change takes effect June 16. Save those numbers and the exact date. Do not assume the first bill will equal either full monthly amount.
Read a worked changeover statement
If the provider charges the first 15 of 30 days at the old rate and the last 15 at the new rate, the example lines would be $15 and $24, for $39 before other charges. That is an illustration of one calculation, not a promise about how your provider handles partial months. Compare the dates and method it actually lists.
Look at the next full period
If a household changed plans halfway through June, label the June statement as a changeover month. Review the first full period separately. If the change confirmation gives no effective date, ask the provider for it rather than guessing.
The next full statement should be compared with the confirmed new terms. A one-time setup fee in June should not be treated as part of every later month's cost. If the July amount still differs, ask about the exact line instead of repeating the whole plan-change request.