So you have a list of numbers like
1
2
3B
4B
and when you're done, you want
1
2
999
999
or do you want
1
2
3999
4999
If you want the second, then selecting the range and doing edit|Replace seems
like it should work.
If you want the first, how about an alternative?
Select the column
Apply Data|Filter|autofilter to that column
Use the dropdown arrow and choose Custom
Contains B
Now select those visible cells
type 999
and hit ctrl-enter to fill all the visible & selected cells with 999.
then Data|Filter|autofilter to remove the autofilter and see all your data.