worried if iphone will get locked if i upgrade my ios 4.2.1

I bought an iphone4 from USA (without contract) in 2010 with the ios 4.2.1 (8C148) MF 03.10.01 and using in India with gevey sim. Now most of the apps were asking for ios 4.3 version for which i have to upgrade my iphone. but I came to know from someone that if I upgrade it with ios 4.3, my iphone will get locked again. no clue, what should i do? should i continue using ios 4.2.1 with limited apps? please help