I posted this under the Contractor topic as well, but thought that it might generate some answers here as well. SMP stands for "Supplementary Man Power". These are contract employees working at/for Saudi Aramco via a third party contracting company. Very different than being an "employee" of Saudi Aramco.
I am a forced retiring USA Aramacon (age 60). Extensions were not available, so I opted for a conversion to SMP as a Plan B.
Rumor has it that US citizens have to bounce out (I already have my final exit visa) for a period of time whilst the Iqama sponsorship switches hands. I can't seem to get a straight answer on how long though.
I am in the exact opposite situation. Been an SMP for 4 years now, been offered regular. In terms of daily work environment, nothing changes. I dont know if Aramco is going to issue you a contractor card or a retiree card. You should still be able to access the camp, but in terms of housing and all other benefits, SMPs are on their own. The monthly overbase pay should cover for these. You are free to choose housing outside.
As for visa, you could be asked to go on exit visa from Aramco and come under the sponsorship of the assigned contractors visa.