I am switching my less than 50 amps manual kiln from 208 volts to 240 volts. What do I need to do to the kiln?

All the elements will need to be replaced. For example, the ohms on the J230 208 volt element circuits should be around 14.2. If you put 240 volts to each circuit it will pull around 17 amps per circuit- which is too much. The infinite switch used in manual L&L kilns can only handle 15 amps. Some older J models, DaVincis, Ovals, B, SQ, G models were made with 208 volt infinite switches. It has been proven that 240 volt infinite switches work fine at 208 volts, but it has never been determined if 208 volt switches work fine at 240 volts. L&L's official recommendation is to replace 208 volt infinite switches with 240 volt switches when switching to 240 volts. When switching to 208 volts from 240 volts a switch change is not necessary. Changing voltage sometimes means pulling more amperage. Be sure to check the electrical specs for your kiln as it will be after the change- i.e. if you are changing to 240 volts look for the electrical specs your kiln as a 240 volt kiln. Be sure your breaker and wire size is large enough. Breaker size is determined by the new amp rating of the kiln, multiplied by 1.25 (125%) and then rounded up to the nearest breaker size. Wire size is tied to the breaker size:

20 amps
12 gauge copper
30 amps10 gauge copper
40 amps8 gauge copper
50 amps6 gauge copper
60 amps6 gauge copper
70 amps4 gauge copper
80 amps3 gauge copper
90 amps2 gauge copper
100 amps1 gauge copper
125 amps1/0 gauge copper (pronounced "one aught")
150 amps2/0 gauge copper (pronounced "two aught")
175 amps4/0 gauge copper (pronounced "three aught")
200 amps250 MCM copper wire (MCM = Thousand Circular Mils)
225 amps   
350 MCM copper wire