Diversity is determined as of the date the action is filed. It does not matter if the parties move after the action has commenced. For living persons the determining factor is domiciliary status, i.e. where you are and intend to be indefinitely. See Mas v. Perry.

According to Celotex Corp. v. Catrett, 1986, the burden on the moving party to produce evidence to refute the non-moving party’s claim may be discharged by pointing out to the District Court that there is an absence of evidence to support the non-moving party’s case. The party moving for summary judgment has the burden of establishing the nonexistence of a genuine issue. [...]