If "all identifiers" would work instead of "all fields", then sed will
do what you want since Java has no keywords with underscores.  
Otherwise, I'm not aware of a smarter tool to do this.

That is called refactoring.

Eclipse can do it but you have to rename them one-by-one.  Eclipse will
fix all the references to that variable across your classes, but you
still have to tell it what you want to change.

if you've stuck to a naming convention that won't cause collisions if
you just removed all the underscores, you can just do a search and replace.

    replace " _" with " "

google for refactoring, there may be tools that do this for you.

   If you're familiar with parser generators (e.g. SableCC, JavaCC, ANTLR,
etc.) it's not too hard to write a custom tool to do this. You can download
a grammar for Java, then use your parser generator to generate a Java
parser. Run the java parser on the source code you want to refactor. You'll
end up with an AST representing your code. Now write a walker that descends
through the tree, and at every field declaration and field occurrence,
modify the AST to reflect the new name.

   This is better than a simple blind search/blind or regular expression,
as it won't touch underscores that appear inside of Strings for example, but
it's not perfect in that it won't avoid name collisions. To handle those,
you'd have to start building a symbol table, and then you're really starting
to re-invent the wheel.

   If you were using Eclipse, you could write an Eclipse plugin to handle
this. The Eclipse does constant compilation in the background, so it always
has the AST, symbol table, and all that compiler stuff ready for querying
all the time; you'd just have to learn the JDT (Java Development Tools) API
for Eclipse to hook into it.

   I'm not familiar with any other IDE to know if they have a similar API
for you to use.

   - Oliver