Is it ok to replace 125v glass fuses with 250v glass fuses of the same rating?
First of all, fuses are designed to protect over-current, not over-voltage. Therefore as long as the fuse amperage is sized appropriately and as long as your fuse voltage meets or exceeds your application voltage the fuse will safely protect.
The best way to remember this, is the voltage listed on the fuse is the maximum rated voltage the fuse can be subjected to. You can never apply a fuse that's max voltage rating is smaller than the application voltage.
You can replace a 125V fuse with a 250V fuse of equal amperage as long as the application voltage is 250V or less. However, the reverse is not true. You can not replace a 250V fuse with a 125V fuse of equal amperage if the application voltage exceeds the 125V.