I made a mistake and find my schematic will not fit on a single sheet. Can I change to a hierarchical model without redrawing the whole thing?
Yes you can. One limitation that you must appreciate with KiCAD at the moment is that it only supports one sheet per hierarchical level Citation this includes the root level.
You can have as many hierarchical blocks at a level that you wish (some #define limit somewhere)
The benefits of a hierarchical circuit is structure and also re-use such that if you have a block used multiple time (gatedrive) and you need to change part of it, you only need to change in one location.
The downside is the navigation can become non-intuitive if the structure has organic growth. Consider re-sizing the sheet before embarking on a hierarchical system
How to convert?
Consider this dummy cct: two cascaded salen-key. You want to convert to a hierarchical system for organisation, re-use or sheet space
I want a common 15.9kHz salen key.
Right-click (or Cntr-V) to paste the circuit into the sheet
All the inputs and outs now need a hierarchical port. Equally if you are not using global power, power connections need ports.
I personally like keeping these pins in the top-left and top-right and then using net names as this provides a clear context boundary for the sheet
- Connect at root Push back to the root and now the hierarchical pins need to be placed onto the symbol click the place hierarchical pins icon and then click onto the hierarchical symbol. This will import each pin one at a time. Place on the left/right as you see fit. as for KiCAD-5.1 you cannot place on top/bottom (would be handy for power...)
Risize as you seem fit. Since I typically do this for re-use v(eg gatedrives), you can link multiple instances to one sheet simply by following #1 and naming the sch file exactly as the one you want to reuse
It kind of depends on what you mean with hierarchical model. Yes as @JonRB wrote you can copy your stuff into hierarchical sheets but just using sheets does not magically translate your schematic into a proper hierarchical design it just converts it into a multi page project.
A proper hierarchical design uses sheets as abstraction objects with a defined interface. This most likely would create quite a lot of work when converting an existing project. (Proper abstraction does not happen on accident)
So going the multi page route might be the easiest way out. (Will still require you to add either hierarchical or global labels to add cross references to your project. Depending on how clean you want to be with your choice of design paradigm.)
A detailed write-up about hierarchical vs flat design (in their pure forms) https://forum.kicad.info/t/hierarchical-or-flat-schematic-design-what-is-best-for-me/16477