I've always heard that if you do go back you should do a complete flush. Not sure if this is true or not. I did do a flush when I switched from regular to synthetic.

 

Modern consumer grade synthetic oils are completely compatible with non-synthetics. How do you think Valvoline, Castol and other major suppliers make a "synthetic blend"? They mix synthetic oil with oil from non-synthetic base-stocks to make the blend.

I think that the "flush" advice started when synthetic oils were first being produced and the producers hadn't worked out compatibility betweeen additive packages yet.