Answer: There is no need to create a new rental agreement unless there is a good reason. A “good reason” might be because the existing rental agreement is old and outdated. But if you are to use a new rental agreement for the existing and new tenant, the existing tenant will have to agree, since technically you cannot “force” the existing tenant to sign a new one.
If that is not an issue, just have the new tenant sign an addendum to the existing rental agreement. I think that is cleaner that having the new tenant just sign and date the existing rental agreement. (This isn’t fatal in just signing the existing document – it is just my preference for a clean paper-trail.)
If using an addendum, all it needs to say is that the new tenant agrees to be bound by all (a) existing rules and regulations currently in place, the existing Statement of Policy, and all pending notices such as rules changes, utility changes, and related documents, which the new tenant confirms they have read and understand. You should make sure that you list all such documents in the addendum, just to make sure there is no confusion by the new tenant about what they are signing on to. You and the new tenant should then date and sign the addendum, giving him or her a copy and keeping one for the file.
As for whether this new tenant will be subject to the rent increase you have planned in six months, the answer is “Yes.” There is a common misconception that MHP landlords cannot increase rent during the first year of tenancy. While it is true that rent cannot be increased during the first year of a non-MHP tenancy (i.e., where the tenant does not own the home, or it is an RV) that limitation does not apply to MHP tenancies where the tenant owns their home. See, ORS 90.600, the MHP rent increase statute. It does not contain any limitations on increases within the first year of tenancy.
Caveat: If you have park-owned homes or RVs in spaces, the one-year prohibition of ORS 90.323 (3)(a) does apply.