Make sure you have glycol/water flow thru the barrel/s before recovering the charge. If not you will most likely turn a simple compressor change into a real mess once you freeze a tube. Other than that, theres no real difference in the changeout.
There may be an oil compatability issue depending on the compressors, trane changed oils awhile back. If their scrolls, you may need to change the oil in the comp your not changing out. Your Trane dealer should be able to provide the SB on the oil.
Your poor planning does not constitute an emergency on my part!!!!