I am a computer engineer from India &amp; want to do my MS in astrophysics either in US or UK.<br /><br />My 1st question would be if this was possible by just taking the general GRE?<br /><br />Or should i take subjective GRE i.e, in physics?<br /><br />Or is it impossible for me to change my stream at this stage?<br />

You should definitely take the GRE General test. Most good U.S. universities will ask for that.

As for the subject test in physics, it all depends upon the university in question. There are quite a few that will not require the physics test, particularly for an MS program (the situation may be different for Ph.d programs). Subject tests can be hard, particularly if you are taking one outside your undergraduate major field, so check first -- it may not be worth taking it until and unless you have to.

There is still time for you to change your focus. But it will help your case for admission if you have taken the first two years of a typical undergraduate physics and math curriculum. I would recommend taking these courses if you can. If you do not have these courses and cannot take them, a strong showing on the subject test might be enough to convince someone to let you in, possibly with a requirement that you take some undergraduate courses first for preparation.