Can I replace a 33uF 100v Electrolytic Capacitor with a 47uF 400v Electrolytic Capacitor?
Large capacitors like that are generally used to filter the powersupply.  As such, a little more won't hurt.  They will be larger, though, so may not physically fit.

You should stop and consider whether you need to replace them at all, though.

A bad electrolytic capacitor will usually be obvious.  

They tend to pop when they go bad.  They usually have lines scored on the top, so they "pop" instead of "bang." 

Look at yours.  If the tops are still flat, then they are most likely OK.  

If they are bulged, or some icky stuff has leaked out then they are definitely bad.

If they are bad, they may be difficult to remove.

One pin will probably go to ground.  On a multilayer board, that may mean there's a whole layer of copper acting as ground.  In that case, it will take a lot of heat to get them out.

What I do is to get out a big 150 watt soldering iron with a big tip - big enough that it can heat both pins at once.  

I put MORE solder on the pins, and heat them both.  After a while, the whole thing is hot enough that the solder melts on all layers.

You can then pull the capacitor right out.  Keep the heat heat on the pads until the capacitor is all the way out.

Replacement goes the same way, but remember that electrolytic capacitors are polarized - you MUST install the new part with the polarization markings exactly as the old ones were.

Lots of solder, heat both holes.  The solder will melt all the way through, and you push the capacitor in and the pins all the way through.  Keep the heat on the pads until the capacitor is seated in its final position.

Check each connection (eyeball them) and resolder each individually if needed.

Clean up any solder splatters with the soldering iron.

Use alcohol and a stiff brush to remove left over flux.