Do you need to remove the old compressor oil when converting to 134A from R12 Freon and refill with new oil compatible with134A?

Yes! You may here many people say that it doesn't hurt to mix these oils. However, they are not compatible and this will damage your A/C system over time. Many people that I know have mixed these oils and over time the A/C's compressor went out. Furthermore, the system needs to be retrofitted. These retrofit kits can be purchase at many automotive parts stores.