Oil change - will this cause issues?

[PieEater]

My understanding is that under the same conditions, a fully synth oil will degrade slower than a semi-synth oil, which will degrade slower than a pure mineral oil.......

Yes but as Mike pointed out a couple of posts earlier it is more complicated than that, a 10w40 semi synthetic which is what most people use in their bikes will be less effective as a lubricant than a 15w40 semi synthetic oil from the get go and will also need to be replaced sooner.