I know our local SPCA also has a long questionnaire(great) but they also don't have the budget

to do fallow ups.
If Otto is fairly well socialized, I wouldn't worry about him losing his status. I have been in the same situation you are in, two separate times with a Germany Sheppard and a Lab, when I introduced a new younger dog(male or female) the older dogs seemed to get kind of a boost, more playful and active.
If you do adopt a more mature dog make sure you keep a close eye on them until you are sure there is nothing to worry about.
Don't worry so much, dogs are very adaptive with the right leadership...you! And it sounds like you know what you have to do just by being here!