I fear it actually isn't even as long as that. Price changes impact on 01.01.XXXX and I believe there is a two-month period when old tickets can be used and/or exchanged by paying the uplift for new tickets (which is usually only a few cents).
After that "Rule of Thumb" comes into play - use/stamp your old ticket and keep your thumb over the price bit if you have to show it to a conductor/inspector...