How to gain another 1536 Bytes Flash on the nano..?

Flash the bootloader for an uno on it!

Normally the nano has 30720 bytes of flash available; the uno has 32256 bytes of flash. The uno exists in two versions: either with DIP or with soldered PLCC as the nano has.

For a howto see
https://www.arduino.cc/en/Tutorial/BuiltInExamples/ArduinoISP/#use-arduino-as-isp

Edit: I Forgot to mention - From now on in the IDE you have to select the uno for programming the (ex)-nano, and you should tag it as "uno" too.