The more elements to be changed, the less the feasibility. For some changes, it has been found that a cost effective solution was to remove an element and replace it with a new doublet. In another case the solution was to replace part of the viewing system with a new beamsplitter and focussing elements. In both cases these existing optical systems were fairly expensive so that the cost of this complexity was less than designing and manufacturing a new system. Aspherization is the new buzz word for refurbishment work.
Aspherized elements can be a great improvement for certain systems.
This is particularly the case for both the Mid and Far Infra Red where many of the materials can be diamond turned to complex aspheric curves. Understand that the replacement of spherical element with an aspheric element will always require optical redesign. One feature of aspherization is that with clever optical redesign, often it will be possible to both improve the performance and reduce the number of optical elements in the system since the correction of secondary spectrum and higher order Seidel factors can be achieved without the requirement of additional surfaces. In the visible, while some machinery is now available for aspheric polishing, most aspherics are still made in the conventional method and are labor intensive. Therefore the same cost philosophy applies, and aspherizing an existing system is more feasible for expensive systems or those with confined access for which conventional optics will not fit.