You will not stay LLC or S after taking a round of funding. But you won't keep your C corp either, because there are lots of details you will get wrong in forming it. It is very straightforward to convert a sane LLC to the C corp that investors want. It is not as straightforward to convert your C corp to theirs.
The main reason being that investors do not want to deal with the pass-through income associated with an LLC or S-Corp.
(Of course, this is not legal advice, and you should speak to your lawyer.)