I believe it can be, but given the volativity of the Stock Market at the moment, why would you want to do that? You need to have a clear focus on your financial objectives. You will need to consider all of your circumstances, The low interest rates of 3.0%ish for some Cash ISAs is attractive for Higher tax rate payers, when considered against interset rates on standard savings accounts. Take professional advice!
Yes it can, but you can't transfer back the other way,i.e. Converting an Equity ISA into a cash ISA.
As has been pointed out, the stock market is very volatile at the moment, and unless you intend your ISA savings to be untouched for a very long time (and a minimum of five years), investing in the stock market is generally not advised.