Begin with scent, days before the move
Rats read the world nose first, so your partner should exist as a smell before they exist as a person in the rats' room. In the week before the move, have your partner sleep in an old t-shirt, then leave it beside or draped over part of the cage — not inside it, where it will simply be dragged into a nest and shredded, though some shredding at the edges is fine and even useful. By moving-in day the rats have been breathing this new human's scent around the clock and it has been filed under home rather than stranger.
If your partner is bringing furniture or belongings into the rat room itself, stage that too. A new sofa arriving the same evening as a new person is two changes at once; bring the furniture first if you can, let the rats claim it during roams, and let the person be the only novelty on their debut.
Still evenings in the roam room come next
For the first several free-roam sessions after the move, your partner's job is to be furniture. They sit on the floor or the sofa in the roam room, treat tub within reach, and let the rats approach in their own order — the bold one within minutes, the shy one possibly not until the third evening. Treats are offered on an open palm at floor level and the rats take them or leave them; nobody reaches, grabs, or follows. Most rats work out quickly that the new statue produces food, and investigation turns into climbing, and climbing into the first tentative sits in a lap.
You stay in these sessions too, doing your normal roam-time things. Your calm is the rats' reference point: if you are relaxed with this person in the room, the room is safe. Keep sessions the usual length and end them as usual, so the routine itself tells the rats nothing has gone wrong.
Teach the safety rules as house rules, not rat facts
The real risks of the arrangement are not emotional but physical, and they belong to the human. A new partner needs the free-roam rules in plain form: shuffle your feet in the rat room during roaming hours, because rats cross floors silently and fast; look before you sit, every time, and pat the sofa cushions down before your weight lands; never reach down and scoop a rat from above, because a hand descending like a predator's is the one thing that can make a settled rat panic; and treat the room door as an airlock, closed behind you, every single time.
Add the room's chew map to the briefing — which cables are trunked, which corner the rats are allowed, what to do when a rat disappears into the sofa base (answer: sit down, open the treat tub, and wait, rather than dismantling furniture around a hiding animal). And agree the hygiene basics: hands washed after handling, no kissing rats or sharing food mouth to mouth, however charming the bonding gets.
Handling follows the rats' pace, then solo duty
Being climbed on is not the same as being ready to be picked up by new hands. Let your partner start by stroking rats who are already out and confident, then by scooping with two hands from underneath, briefly, during a roam, with you beside them. Rats who flatten, squeak or run from the new hands are voting no for now, and the vote stands. Within a few weeks most partner-rat pairs are at full handling, because the rat has dozens of calm repetitions banked.
Solo roam duty comes last: your partner runs an evening alone only once the rats come to them willingly and they can name who is who, who hides where, and who must be watched near the door. Adding your partner to your rats' KinsLe profiles through Family Sharing gives them the whole file at once — names, ages, quirks, health notes and the roam routine — so the knowledge you carry in your head is in their pocket before their first evening in charge.