What actually goes wrong
Not the new provider being worse. The damage comes from the gap: campaigns paused before replacements are live, tracking rebuilt from scratch so history breaks, audiences lost with the old account, and two weeks of no advertising during the handover.
Every one of those is a sequencing problem, not a vendor problem.
Before you give notice
Confirm you own the ad accounts, pixels, audiences, and creative. If you do not, that is the first negotiation — and the reason to have it in writing next time. See how to check who owns your ad accounts.
Export ninety days of performance data yourself. Screenshot the account structures. Note your current cost per lead by channel so you have a baseline nobody can dispute later.
The parallel week
Give the incoming team read access while the current campaigns keep running. They rebuild in parallel — same accounts, their structure, their tracking verified — and nothing goes live yet.
Then cut over at a month boundary: new campaigns on, old ones paused the same day. No gap, no double spend, clean monthly numbers on both sides of the line.
The sixty-day rule
Do not delete anything for two months. Keep old campaigns paused rather than removed, keep the old provider’s access revoked but the account intact, and keep your exports.
You will want the comparison, and occasionally you will want a piece of the old setup back.
What to judge and when
Month one is transition noise. Month two shows direction. Month three is the number.
Judge on cost per booked job against the baseline you wrote down, not on how the reports look or how responsive the new team is in week one. Everyone is responsive in week one.