First, the former firm should be asked for permission, which should not unreasonably be denied.
As licensed professionals, architects are always personally liable for their own negligent acts, errors and omissions. Therefore, if an architect is the sole owner of the corporation or LLC, there is no real benefit in the form of legal entity.
Most state licensing laws are both “practice” laws and “title” laws. Practice laws define the practice of architecture and say that you must be a registered architect to offer such services.
Collaborate provides no information about the legal relationship between the parties. Associate is not much better because that could imply any number of different legal relationships. On the other hand, a joint venture is when two parties form a partnership for a specific purpose, often a specific project.
There are options depending on the size and nature of the Project. One choice would be to start with AIA B101-2007 or B104-2007 and modify them to delete services that precede the Construction Phase.
Unless this is covered in the contract, there generally is no specific time period. Statistically, most professional liability claims are filed, if ever, within 3 years after substantial completion of the project.