Standard cost update program versus inventory period closure

We are having standard cost uodate program planned, however inventory period for earlier month is not yet closed.

We are having some cocerns here on the same;

1. Can we start cost update program when inventory period is not closed. If not, then why ?

2. Can we perform transaction when earlier inventory period is not closed but dont want to use OLD frozenn cost. If not, then why and if yes then wont it have any difference in new trnasctions cost and ealier costs ?

3. What are impacts, if we perform the transactions now after closing ealier period and will perform cost update in coming week ends ?