It depends on what materials you already have. The €100 buys you the piccolo reeds, but you still need wax, a melting pot, spoon, soldering iron, maybe also some valves, bits of wood and glue...Paul, thanks
So if i could do it myself cost would be about €100 plus registers
if i paid to have it done approx €1000 including registers and tuning, is that a fair guess?
Guerrini oxford 3I could be wrong but I don't think it will be possible to alter the registers to give you the Reed combinations you might like to have. You may need to have a new switch installed to activate the H Reeds. As a matter of interest, what make and model of accordion is it?
The LMMM is valued at £300, the LMMH will cost £2600It depends on what materials you already have. The €100 buys you the piccolo reeds, but you still need wax, a melting pot, spoon, soldering iron, maybe also some valves, bits of wood and glue...
And the work on the registers highly depends on which registers you already have. Some LMMM accordions only offer L, M, LM, M-MM+ and LM-MM+ and then it's rather hopeless. Others offer L, M, LM, M+, MM+, LM+, LMM+, M-, M-M, LM-, etc., etc., and replacing M- by H gives you all the registers you need. Sadly, it's more likely that you will be missing register options you want than that they are all there already.
What it will cost to have this change made is hard to predict, but a minimum of €500 and more likely a cost of €1000 is not unreasonable.
General advice is therefore to sell the LMMM accordion and buy a similar one which has LMMH.
300 is a very low value for the LMMM if it is in good condition. (A completely restored good as new goes for more than double here.) And 2600 should be what an LMMH with cassotto goes for. And LMMH without cassotto rarely fetches 1500 here, often considerably less.The LMMM is valued at £300, the LMMH will cost £2600
Accordion prices do go down considerably with age. That Fisart is a fairly recent accordion. And the store you link to always lists ridiculously high prices compared to the competition. Strange business model. The store's description of the now defunct (sold) Fisart company is also "not exactly accurate" to say it very politely.