FAQ: Will there be a transition period where both old and new transactions are possible?

Probably.

There are technical reasons why you just can't fully stop to support "old" transactions and only process Seraphis transactions from one block to the next: When we reach fork height there may be a number of transactions still waiting to get mined in the pool that we shouldn't simply drop as suddenly "invalid" or "unsupported".

In earlier hardfork there was a transition period of 1 day where transactions both in old and new format were valid and processable.

There have been ideas to make this transition period much longer in the case of the Seraphis hardfork, e.g. 1 month, to give people more time to adjust. This would probably have advantages as well as disadvantages however, and it may need a careful analysis to weight them against each other. I have written down some early thoughts here.

Note that a transition period does not mean you will be able to mix the use of old and new addresses, and no pre-Seraphis wallet app will be able to send funds to a Seraphis address, regardless of the length of the transition period.