Do not use r-12!!! R-403b is a replacement for R-502.

Do you have R-402a or can you get it? If so this is the recommended refrigerant by TK.

You will have to change the compressor oil with TK part number 67-404, you will also need to change the compressor oil filter if equiped along with liquid line fiilter drier.

Evacuate the unit to 500 microns and charge to 90% of recomended weigth on the unit id tag.

You can use R-404a to as long as you change the oil to a POE tk part number 203-513 at 4 qts. you will also service comp oil filter and drier as well. run unit overnite to collect and migrate oil back to compressor sump and for safe system measures replace the oil and filters one more time to assure you have no mixed oil in the sytem.

I hope this was of help.