The following is with my professional hat on. An insurance contract is a contract of "utmost good faith" (uberrima fides) and goes far beyond an " ordinary" contract of "let the buyer beware" (caveat emptor). You MUST disclose any and every "material fact". A material fact is "any fact which would influence an Underwriter in deciding whether or not to accept a risk or not and if so at what rate" I would suggest that if an Insurer asks a specific question about any point then they consider that point to be "material" Failure to disclose a material fact (whether specifically requested or not) would entitle an Insurer to avoid the contract, though they might not do so. So, in short, disclose EVERYTHING. every modification, every change, every sticker on your panniers! That is the counsel of perfection, but really not very practical, I know. So get to speak to someone in your Insurers/Brokers who knows at least something about (classic) bikes and at least something about Insurance Law, and tell them everything, asking if they think it material. (When I was asked about modifications to my Egli Comet Special, my Insurer accepted my explanation that it was a one-off special, and nothing was standard). SOME Insurers MAY be looking to avoid claims, but what most really want to know is EXACTLY what they are insuring. Many people think that "fiddling" insurance is a legitimate pastime, so they have to be precise. Only you really know your bike/history/driving experience/attitude to insurance/etc. so tell EVERYTHING, and you can't go wrong. I have 10 bikes ALL of them modified some way or other so I have it all recorded in writing and just send it to a prospective insurer rather than wade through it verbally every year. Finally I completed my Chartered Insurance Fellowship more than 40 years ago, and although basic Insurance Law has not significantly changed, the public attitude to Insurers, and the attitude of Insurers to the public has changed. If the public are not completely "straight" with Insurers how can they expect Insurers to be anything but suspicious,It may be a long winded nuisance to tell an Insurer everything "material", but a legitimately avoided claim is a b----y sight more nuisance.