Similar to Stephen's answer, but slightly different in the way it's executed:
sed -i '/^RELEASESTATUS=/s/$/xyz/' keyvalue.txt
Instead of replacing the entire line with a copy of itself with some text added to the end, this first locates the correct line (the line that has
RELEASESTATUS= at the very start) and then substitutes in the required text at the end of that line.
This uses the fact that the general form of the substitute command (
end gives the range of lines that the command should be applied to, given as a start and end address (and "address" may be a line number or regular expression, or an offset). When using a single address, the command is applied to that line only (or to the single lines matching a particular regular expression).