Update README.md

README.md

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 ### They should enable image2image for ANY model trained or merged from the Stable Diffusion v1.5 base model.  They will not work with models derived from Stable Diffusion v2.0 or v2.1 base models.<br><br>
 
 ### INSTRUCTIONS
+- If the model folder that you are upgrading already has a VAEEncoder.mlmodelc file inside, rename that file to VAEEncoder.mlmodelc.bak first, to keep it in case you want to teturn to it later.  It is fine to leave it in the folder.  It only needs to be renamed.
 - Copy the appropriate new VAEEncoder.mlmodelc to the model folder of the model you are upgrading.  The new VAEEncoder.mlmodelc file needs to match the original model in size and compute unit type.
 - Examples:  If the model is an original 512x768 model, copy the file named NEW-ORIG-512x768-VAEEncoder.mlmodelc.  If the model is a split_einsum 512x512 model, copy the file named NEW-SPLIT-512x512-VAEEncoder.mlmodelc.
 - After copying the appropriate VAE file, you must rename it to VAEEncoder.mlmodelc.  That means removing the NEW-ORIG-512x768-, NEW-SPLIT-512x512-, etc., from the copied file's name.