I become a vegan for Lent (the 40 days before Easter) every year so I'm currently a vegan. I'm pretty used to it by now but there are definitely noticeable changes when I alter my diet. As long as you manage your diet properly though, your energy shouldn't be any different.
There are a few notes about going vegan. Try and take iron and B12 supplements, especially if you plan on doing it long-term. Also, try and have a diet that includes a breadth of variety, including fats. A lot of vegans drop off in energy and lose a lot of weight because they neglect fats which are an important energy source. Try and experiment with medium-high fat foods like avocados, coconuts, sauces and curries and use oils in your cooking. However, that doesn't mean to include foods high in saturated and trans fats like deep-fried foods, etc.
Also, before you start experimenting, get an idea for the sorts of recipes you want to make. The first week or so can be pretty hard as you can run out of ideas pretty quickly which can make the foods you are eating get boring. And learn how to read the nutrition information panels on packaged food. I don't know if you are planning on trying vegan or not, but there is milk and eggs in nearly all processed foods so you need to be careful.
I'm a studying to be a dietitian (been at uni for 5 years so far) so I have a fair idea of what I'm doing, but if you're not sure try and talk to someone who can help you.
Oh and if you want to go back onto meat after your experimentation, do it gradually. Scoffing down meat (and milk and eggs) after being a vegan for a month and a half is the worst feeling ever EVER!