What do you have to migrate? Custom portlets? What features of Websphere are you using?
You need to determine what features of Websphere you currently use - that includes portlets delivered with Websphere, authentication setups etc.
Also, you need to check your custom-developed additions (e.g. portlets) and how much of IBMs proprietary additions to the standard they are using.
And you need to determine what Liferay features you will use to replace your currently used functionality.
Basically: This question is far too broad to be answered in a simple forum post.
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