Upgrading Existing Spells

[Felix]

The PHB talks a little about how wizards can research new spells, at the cost of 1000gp per week, with a number of weeks per spell level. It generally leaves the assignment of spell level up to the DM and the player. It doesn't say if upgrading an existing spell, with the exact same spell effect modified in some aspect also falls under this 1000/week/level umbrella.

Example:
Detect Thoughts.
Duration: 1 minute/level.
Upgrade: Duration 10 minutes/level

Ok, so is this change going to run me the full cost?

Secondly, is there somewhere that provides guidelines for how many spell levels changes like this would require? Is a "Greater" Detect Thoughts a third or a fourth level spell because of the change from 1min to 10min?

 

[Pax]

It'd be an entirely new spell. With a longer duration, it'd not be just a better version of Detect Thoughts; it'd be Greater Detect Thoughts, and would probably warrant some other small benefits, too (like getting the 1-round-concentration-time results without spending an action to do so). What that means is, it's the same as creating the spell from SCRATCH. And in turn, that means ... yes, Virginia, you have to pay full price to develop the spell.

As for how many levels it would change ... no, there're no guidelines other than "examine the power of the new spell, and see which level best fits it".