To put it as mildly as possible, yes there seems to be a probability that the vet conversion rate is not increased to 200%. I bought three days' premium (so conversion rate 300% in total), and tried to take three engies lvl 42 from 50 vets to more...
Result: 28 tries (280 experts) yielded 40 vets in total, giving one engy 20 vets more and the other two 10 vets more each.
For all us superstititous players, I did that after a won battle where I made nice damage.
For sailors that low and for that low number of vets, this is ridiculous.
I don't feel like converting anymore - this looks like daylight robbery.
Little calculation: 4 conversions out of 28 tries gives a 14.29% success rate. Divided by 3 (for the event and prem) gives a 4.76% base success rate for a lvl 42 sailor with 50 vets. That is really shoddy.