Something more than playing with denominations may be afoot here. Last September, the regime devalued the sum in line with the black market rate, destroying the black market and potentially setting the stage for convertibility. Inflation, historically 6 to 7% seems to have settled below 6%. For a developing country, 5% is a perfectly good inflation rate.
Consider the euro equivalent value of the proposed coin denominations using today's rate of 100 000 sum = EUR 9.98991: UZS 10 is 0.1 eurocent, 50 is 0.5 eurocent, 100 is 1 eurocent, 200 is 2 eurocent and 500 is 5 eurocent. With 5% inflation, only the 500 sum seems sustainable in the future.
However, comparing prices (not wages) when I was in Uzbekistan last spring, my conclusion was that the PPP (purchasing power parity) rate should be (very) roughly around 5500 sum to the euro, compared to the official rate of 4200 and the black market rate of 8000. If so, the official devaluation was far too big. Either the sum should rebound if it is made convertible OR the authorities wanted to create a large artificial advantage for Uzbek exports. The latter option seems more likely, as the main export article is cotton and the country's cotton has a bad image because of child and slave labour charges.
If I am correct in the previous paragraph, it is unlikely that we'll see the 10 and 50 sum coins and even the 200 seems pretty doubtful, because it would incur startup costs.
Peter