Maybe this would help: I remember an occasion when a French colleague of my husband's came to our house for dinner.
As colleagues, he and my husband were on 'tu' terms. But he had never met me before, and he addressed me politely as 'vous'. Then, at some point during the meal, when we had relaxed, got to know each other and had a glass or two of wine - between the main course and dessert - he switched to 'tu'.
This switch seemed natural to me. It might have seemed over-familiar to have begun the evening calling me 'tu', but, as we were meeting on social basis, it might have seemed unfriendly to have called me 'vous' throughout the evening.
Could I ask any native French speakers if that's what normally happens in such a situation?