Why are Multiple Theme Songs sometimes listed for a single Series?
Although a few programs are associated with a single piece of music -- think of the Lone Ranger's use of the "William Tell Overture" for example -- sometimes a radio or TV series has more than one THEME during its run.
So why would the THEME change? There are several reasons:
#1.) The producer and/or sponsor has decided the show needs to be "freshened", and changing the music points up the fact there has been a change of setting or cast. This may be an attempt to boost ratings along with other changes.
#2.) The series ends its run on one network, and some time later (sometimes after a gap of a few years) the series reappears on a different network or in syndication. Again, the producer, sponsor, and/or the network music director think that a change of THEME would be more fitting for the "new" version.
#3.) An external circumstance necessitates the change. For example, an organization called ASCAP (The American Society of Composers, Authors and Publishers) has collected performance royalties from radio stations and networks since the 1920's. However, there was an "ASCAP Strike" starting in January, 1941. This was a boycott of ASCAP over its doubling license fees in 1940 by radio broadcasters affiliated with NBC and CBS. This meant that those radio networks and stations had to change all music to non-ASCAP sources. So if a piece of music (including program THEMEs) had an ASCAP-affiliated publisher, it had to be replaced by January 1, 1941 so to avoid the fee increase.
In reaction to increasing ASCAP fees of 448% during the previous decade, in 1939 a rival performance royalty organization had been formed called BMI (Broadcast Music Incorporated.) So some publishers had already switched from being affiliated with ASCAP to BMI. But for other publishers who remained with ASCAP, their THEMEs had to be replaced on NBC and CBS networks, stations and related program syndicators by Jan. 1, 1941. The boycott lasted 10 months, so in October, 1941 ASCAP agreed to reduce or delay its rate increases, and broadcasters again decided to use ASCAP material. But by then, many THEMEs had already changed.