If $L$ is a regular language, then $s(L)$ is also regular
First thing you must understand  is that regular expression are closed under union(+), closure(*) and concatenation (.)  . reason being every regex denotes a regular language and regular languages are closed under union, closure and concatenation.

Second thing you should know that if r1 and r2 are regex then the following regex are also regular


r1 + r2
r1.r2
r1* ,r2*


so if you are substituting a regex by another regex it should fall among the above three categories or in simple words you will be able to form the regex(after substitution) with use of above relation. coming to you question the language generated after substitution can be formed using above closed properties over regex. let say r1 = a , r2 =b  then we have 

( ((r1.r2) * ) . r2* )  making it regular

one more thing the only operation defined to regex are the above three  and if so any substitution of regex by regex will have
either of the three operation associated with it thus making it regular again