You cant cash it in. You can either transfer to new employer or to a PRB which is a pension bond. You can presumably leave it where it is as long as the pension scheme is still active but I suspect that the new company want to close down your original comp[any pension which is why they are asking you to transfer. The transfer is fairly seamless and it helps to keep all of your pension within your current employer. If in 20 years time you try to contact the previous scheme you may have problems getting trustees names etc.