Gare de Lyon to Gare Montparnasse

to do that connection quickly and easily without connections isn't easy. Métro would have been a hassle, Montparnasse isn't on the RER, and the non-change 91 bus involves a lot of walking.

Are you up for that much walking?

I rarely use taxis but I think I would in this case,,,

Another vote for a taxi. It should not exceed 20-25€.
If you must take the bus, you have to walk all the way to the corner of rue de Lyon and Boulevard Diderot to find the 91 stop. Depending on your arrival platform, that's a 5-12 minute walk. I am unclear about the current status of on-board ticket sales, I think you can still buy one, but cash only (2,50€) and exact change recommended. Else, head to the metro station to buy a ticket at a machine there...

A regular t+ ticket costing 2.10 EUR works for the bus. I don't believe they have tap and go. There is a system to use your phone, but you need a local Paris phone number for that to work, as the bus fare is automatically paid on your phone bill. Costs 2.50 EUR as well, more than a t+ ticket.

You can usually byt T+ tickets at the bar in the TGV, so if you are taking a TGV to GDL you can do that. In my epxerience taking the bus 91 is a good option. It is a bit of a walk, yes, but not more than than the metro would involve, and there are no steps This bus runs mostly it its own right of way, and is thus relatively quick.
If you take a taxi it will take the same buslanes, and will probably be behind a 91 all the way...

The bus stop is across the street and one block further than the taxis. We took it in June. One note: we stood and held onto our tiny suitcases the whole way. It was pretty crowded although it was the middle of the day. You wouldn’t have to stand in a taxi.

Thanks for all your helpful replies!
I will have departed from Thun Switzerland, so stretching my legs before yet another train trip to Chartres will be just what I need. Besides I need to be ready for RS Best of Paris. I'll buy my bus ticket on Lyria and take the 91. Here in BC we have courtesy seating, so I'm hoping that since I have white hair I might be given a seat rather than having to stand the whole way.