The only place I have ever heard that is on online drug forums like Bluelight. When I used to inject heroin I would occasionally do a shot that had been prepared the night before and I noticed no difference in effects or potency.
This is the best answer from a scientific source I could find:
Basically I think it is saying that it converts to 6-monoacetylmorphine, not morphine, that it converts very slowly (only 4%-13% was changed in 24 hours), and that how fast it hydrolyzes depends on a number of factors, including the acidity of the solution. 6-monoacetylmorphine is actually
more potent than heroin (see:
http://www.ncbi.nlm.nih.gov/pubmed/6543399) . Your brain converts heroin into 6-monoacetylmorphine and then to morphine when you inject heroin anyway.