Stop worrying, be positive, have a life rich with experience, do not believe everything you read or hear, especially from aliens.
It may be that we have to prepare our bodies through the use of chakras, but the alien did not tell you how. Rather important omission, don't you think. Reki, which you mentioned in a previous post, seems to be a viable method for healing. So I expect that its processing of the chakras is also viable. So why not just use its methods. Afterall your path brought you to Reki, probably by coincidence. That seems to be the method of creating pathways for us to follow. So do not muddle it up with negative thoughts and concerns that you have to do something right away. When you try to make things happen yourself, you tend to get off the path intended for you. So I recommend just passively watching what comes your way (without you making it happen). Your job is to take advantage of the situations that emerge.
In my example, I did not make the 96 chakra system happen. It came to me through a series of coincidents plus some observations on my part, plus use of my math & physics background to make a synthesis of the bits and pieces I was getting by coincidence. That seems to be my path, along with a life rich in experience. So when I pass away, if there is something else, I'll have something to offer. Or perhaps as Shakespear(sp?) said, life is but a stage. Perhaps our lives are just entertainment for some other beings. Either way, make sure your life is full and entertaining. I think of life as an audition for a committee position in the afterlife.
By the way, the fourth dimension is time. So one interpretation of your aliens message is that since we are now living in the time dimension, we will soon be leaving the time dimension. That is we will die. Nothing new there. It's going to happen to all of us sooner or later. So the alien is telling you to get ready for it. The alien happening, by the way, I presume was a coincidence. So it might have been a means to guide your pathh. It certainly aroused your concern.
The evacution word is interesting. It reminds me of the born-again christian concept that at some point in time, god will take all the good people and leave the remainder on earth. In the past I've speculated on what physical mechanisms could account for such a thing. My conclusion is that radioactivity could do the job. In Hiroshima there was a Zen Macrobiotic hospital very much in the radioactive zone where not a single patient got radioactivity sicknes, so I have read. So a nuclear war could be the mechanism to separate good from bad, and the right diet along with a positive attitude could help you survive. The one problem with this logical process is that its the bad that survive.
So bottomline, I can tell you what I think. But you had better do your own thinking for yourself. If there is a god, it's a god of diversity who would want everyone to find their own unique path and personality.