Dont think there is a set date for refs not acknowledging the old numbers, however some of them you come across are becoming a joke and it's kindof ref discression at the moment as to whether they will walk up to you with gaffer tape and remove some of the random letters / symbols so it just says a number

As Justin said there is no set cut off, but the change came in play on the 1st of December.

Some stuff refs will ignore, to use your example
NOT acceptable
5.0.8 would be called 508
4-5-6 would be called 456
X would be called 10 if there is another 10 on the team reprint required
XIX would be called 19 like wise
weird symbols like £ or ? well we hate them anyway reprint required
80008 to many digits under new rule reprint required
10/6 would be called 106 we have the power to ignore symbols especially it the are 2" or smaller
etc

Remember numbers have to be at least 4" everything else smaller.

What it boils down to though for this and subsequent bouts is what was a greed in the contract. If you said we will bout to 4.0 then you are good, just tell the refs. If you said we will bout to whichever rule set is in force 4 weeks prior to the bout you have inadvertently agreed to honour the changes. However if both teams agreed to waive it then you are good.