Ahoy!

So.. I've been lifting nearly 4 years now, and seriously (3-4x pw) for three and a half.

As expected my gains have slowed down somewhat. Over the past six months i've started to shrink! My arms have dropped from nearly 17 to 16", this is uniform with other body parts.

My question is, should I adapt my training programme radically? - from what I understand I am currently following an intermediate routine working body parts 2x per week. Should I change more of an advanced spilt?
I have loved the baby got back programme and followed this for about a year now. I have typically always avoided splits. Nearly all of my previous routines have been based around compound movements with little assistance.

It's also worth mentioning with my previous experience lifting very heavy for low volume, 6,8,10 reps is good for me.

How should I adapt my training for this 'transition'?

Thanks in advance for all your help!!