You can just paint over the top as long as the existing surface is clean and lightly sanded to aid in the adhesion of the next coat.

Any paint used with a paint base will work well to cover the existing paint.

paint or acid etch

As long as you are using a similar type of paint (I expect it is an indoor latex, since baseboards are found indoors) and as long as the existing paint is still in good condition and is not flaking off, then yes, you can paint right over the existing paint. If the paint is in bad condition, you should remove it before repainting.

Elastomeric paint can be applied to a surface that has been previously painted with elastomeric paint. However, with each subsequent layer of the paint, the elasticity of the treatment breaks down considerably.