Changing train stations in Paris

E RER at the Magenta station is connected to Gare du Nord (just follow the signs to E). and arrives at Hausmann St. Lazare which is connected to Gare St. Lazarre. I would want 45 minutes if I were not familiar with all this --- to walk to the Magenta station (indoors) and then from Hausmann St. Lazare into the St. Lazare station and find my track. The trip is 20 minutes or so. It costs a couple Euro -- ordinary metro ticket. RER trains don't run as often as metros, so you might have as much as a 15 minute wait. The stations are listed on a board over the track so you can see that you are going the right way and that your stop is listed.

I would not do this by cab as traffic in central Paris can make that very slow -- definitely take the train.

It's one stop on the RER E, so as long as you avoid rush hour it should be a pretty easy connection.

How about Est to Montparnasse stations?

For Gare de l’Est to Gare Montparnasse: Metro line 4, 20ish minute train ride (13 stops) followed by a 750ish meter walk.

They are the 'same station' in the same way Nord and La Chapelle are --- connected but a bit of a walk.