Apologies if this has been covered earlier in the thread, but an industrious French rugby fan called vino_93 did work to identify how many active players would be available for switching in time for the next RWC (based on information that is freely available, there will obviously be more once people start checking birth certificates etc).
In terms of active players who have represented Tier1 nations the figures were
Tonga - 13
Samoa - 13
Fiji - 11
Then there is a sharp drop to Algeria (4 or 5 frenchmen), then the likes of Portugal (2 or so).
It remains to be seen how many players would willingly switch, but I imagine this will happen if agents believed RWC exposure will boost their clients wage potential. My concern is this may create an entrenched gulf between the PIs and other Tier2s who do not have a history of emigration and have to solely develop their own players. It is worth noting that the combined Tongan, Samoan, and Fijian populations of Aus & NZ are all larger than the population of the islands themselves. So this will be a gift that keeps in giving for the islands for a minimum of two further generations (i.e. until at least 2041).
It would be fair to say that I am not a fan.