Problem

You need to replace a string containing double $$ using regular expression by a string containing a double $, e. g. original string $$Reusable_only_switch= replacement string $$Reusable_only_switch=true.

Solution

Put the $$ in a group and make the group part of the replacement string, e. g. regular expression ^(\$\$Reusable_only_switch=)\s*$ replacement expression $1true.

Shortcoming

If the double $ needs to end up at a different position, for the time being I do not see other solution but to take several steps.