I want to report that I successfully converted my FR-300 to FR-301, as per all the information that was discussed here, i.e. with keeping the FR-300 flange and extend it with brass spacers and metal washers.
My biggest surprise was how easy and straight-forward it was - it took me less than 15 minutes of work, but that's just due to the long discussion here and as result of that I was very well prepared.
Costs are tremendous though: 150 USD for the parts, which is over 50% of brand new FR-301 in the USA.
So, IMHO one of the few scenarios in which it's not pointless (from financial point) to do it, is if your FR-300 heating element has failed - then investing in new FR-300 heating element would be waste of money and it's better to buy FR-301 heating element and do the conversion.
Also, if you want FR-301 and you're in places like the USA or Japan, it's better to sell your FR-300, add some money and buy brand new FR-301 instead pay 150 USD and do all the work involved in converting your old FR-300.
In my case it has more value in doing the conversion, because first of all it was cool project, second of all in Europe, FR-301 is very expensive - the only realistic way to buy it, is from USA or Japan (some eBay sellers), but then the custom duties come in place. So, with buying parts - no custom duties and relatively low shipping costs as the parts are small and not heavy. Another reason is that all my FR-300 nozzles were worn out and I did not want to invest in buying more legacy nozzles.