It is a truth universally acknowledged that if you want to run a play for a long time, you will eventually re-cast.
It is one of the downsides of a successful acting industry. If the actors are good, their availability is much harder to come by. So if producers/directors want the “show to go on”, then you must replace actors.
But what are the actors busy with?

Many actors are simultaneously part of six or seven different productions; or even film assignments, which often block actors for large schedules even though they only require them for a few days. The most recent competition comes from television commercials. Theatre actors are often conscripted to be part of “performance” driven adverts like the latest Tanishq commercial.
All four actors are from the theatre. It is wonderful that talented actors are finding lucrative work, but often they get called up at a moment’s notice.
Some directors are smarter. They try and double cast the play while it is being created. So that the busier actors have an understudy to take on their roles in case there is a clash of schedules. Recently Bombay Talkies, a series of monologues about Bombay, seems to be doing this. Often the audience doesn’t know before-hand which version of the cast is going to be available for the show. Akvarious’ Guy Thing, is a prime example.
It is a two character play, but there are six actors that are prepared to go on!
There is a chink, however, in the armour of this logic — usually a busy actor is a strong actor; and a strong actor needs to be understudied by an equally strong actor; who often is as unavailable as the person she/he is understudying!
It is like one of those maths equations that it seems impossible to get your head around.
I am not a fan of double casting or re-casting if I can help it. I am more of a purist — if the cast is unavailable then the show is unavailable. It is not as though we are trying to re-cast Spear Carrier 6, but one of the core cast.
However all the long running plays have had to make adjustments. Class of ’84 has undergone many changes, some stop gap, some permanent. Most recently, they have conscripted their lighting operator Arghya Lahiri into the cast. Vagina Monologues, another monster hit, has also had to adapt.
They regularly play at The Comedy Store. Although originally a five actor piece, they have found their own mantra in dealing with cast availability. As long as three of the cast are available, the show will happen. Parts are quickly redistributed and each performance can have a different combination. In fact, Vagina Monologues is such a well oiled machine, that it often happens even in the absence of the director or producer.
However, the Vagina Monologues team revel in the cast changes. It keeps the show fresh and keeps everyone’s focus much stronger. It is a new challenge that must be conquered and a new combination requires renewed concentration and energy.
I encountered a similar scenario on one of my shows, Nostalgia Brand Chewing Gum, directed by Bangalore based Vivek Madan. We needed to permanently replace one member of the cast. We trudged back into rehearsal. Strangely, we found it an incredibly enjoyable experience.
The other actors too have responded strongly to the new development. Rehearsals are back to being about exploring and figuring things out. They play has changed remarkably. A new actor = new energy + new interpretation.
So maybe “recasting” isn’t such a dreaded thought anymore.