Destroying your current network
Towns won't allow you to destroy more than a couple buildings before dropping their rating too low, even from the maximum of '1000' rating. It's also quite easy to get enough rating back to build a new station by building trees.
Dragging the bulldoze tool across the map after having sold all your trains or put them in depots is thus quite an effective way of quickly getting rid of your network.
Keep in mind that bulldozing water can get quite expensive. You can hold down the SHIFT key when doing any building or terraforming operation to check how much a that operation will cost in openTTD without executing it.
Upgrading an existing network
I've often had to do this in my games. First off, things become a lot less painful if you have made use of the 'groups' system to group all your collections of trains that have the same orders. However, it still features quite a bit of manual steps with default openTTD installations.
If you have not, then I advise you to make a group for each of those collections. You can do this in the vehicle list. It's a bit of organizing that pays off managing your network later. Groups can also have shared orders which means if you change the orders of one train in the group, all the others will copy your changes.
Then, order all your trains to move to their depots from the same vehicle list by selecting 'all vehicles' then 'manage list' to 'go to depot'. Depending on the amount of depots, the size of your network, and the speed of your vehicles as well as your game settings it may take anywhere between a few months to several years of in-game time before all trains are situated in a depot.
Then, you can start upgrading all your rail with the 'convert rail' tool, except for the depots, to monorail or maglev. It can be found in the rail construction menu.
Afterwards, we can build the new trains. For each of your rail groups, do the following, assuming you're upgrading to maglev (otherwise replace 'maglev' with 'monorail'):
- Find the station that's first on the order list of an old train.
- Build a new maglev depot near the station.
- Manually create a single maglev train that matches the railway train.
- Open up the orders menu. Hold down the CTRL key and click on the existing old train in the train list.
- Check that the maglev train has copied orders from the existing train.
- Now click the 'clone' button in the depot, hold down the CTRL key again (for shared orders) and clone your new maglev train any number of times you want. I advise cloning 1 maglev train for every 2 railway trains you had previously, as they are quite a bit faster, more powerful, and carry more cargo.
There are some newGRFs that can make the process a bit less of a chore. I'd recommend you look into Universal Railtype depots that can support trains of every rail type. With these you can replace railway vehicles with maglev vehicles using autoreplace.
If you're playing on a multiplayer server that spans multiple railway types in its time period, consider recommending this type of newGRF to the admins. It can be configured so that the universal railtype has a speed penalty (e.g. a max speed of say 50 kph) so it can't easily be abused and is only used for depots for type switching without having to have amazing apm to keep up.