I dont think its true.
You can switch to any good mineral or semi synthetic with current API like SL, SM or SN any time.
There might be a temporary issue while switching to synthetic after running mineral for a long time with out following proper oil change interva . It is because synthetic contains strong detergent and cleaning agents which will break down any carbon deposits on the piston and cylinder region. This will make the new oil loose its colour and will clog up the oil filter in few days. Piston deposits removed by the new oil also makes way for some combustion pressure to leak out of the chamber or in some cases oil reduction issue may arise.
I hope you got the point.