Lifting - how to change things without losing strength?

I was getting in a rut. Rather than getting stronger, I was actually getting weaker. And I was getting very bored! For example, I had my squats briefly up to 185, but for some reason starting struggling and ended up barely able to do 155 (5x5). And I was doing 17 pushups (3 sets) and then I could barely do 12. Luckily my deads stayed up at 210#, but I stopped advancing - I was stuck!! My bench was stuck too as was everything else I was doing.

I'm assuming that some of the problem could be due to a not so great diet for a while and gaining some weight back. I wanted to change things up a bit and find a new program. So I pulled out my NROL Supercharged and started at the beginning. That's all going fine and dandy, but because I'm doing different exercises and lower weight/higher reps (3x12 right now) I'm worried that I'll lose the strength I had going on my other lifts. I think this program is like 6 to 9 months long and I'm only 3 weeks into it.

My question is this - do you think I should add some heavier lifts with lower reps to the program to keep my strength up (and if so, how/when) or should I just follow the program and not worry about lost strength knowing I'll probably build it back up anyway. I was getting really proud of the weight I could lift and I hate to lose that. But maybe my pride just needs to take a back seat while I start over and build things back up.