UART/I2C Pull up resistors
Bus on 2.5V, pulling up to 3.3V:
It depends on the IC's communicating this bus. When their SDA/SDC input voltages are allowed to become 3.3V, you can pull the lines up to 3.3V. (e.g. some 3.3V uP allow for even 5V input signals). (Do check max input voltage level as well as max input current.)

Bus on 3.3V, pulling up to 2.5V:
Check again the datasheets of the IC's communicating this bus how their logic levels are defined.