Yes. The Gotthard route is open as "usual" (aka all trains take the old scenic route).

For the Simplon route there are replacement buses:

- between Milan and Domodossola: frequent, 13€ ticket and 3€ passholder fare. From there plenty of trains towards Brig and Lausanne.

- between Milan and Lausanne direct: 3 times a day, expensive tickets and 13€ passholder fare.

As far as I know the passholder fares are only available at Swiss (and likely Italian) ticket counters. If you go through Switzerland at some point you could get them. Otherwise a 13€ ticket is the same price as the usual passholder fare for the train...

To get to Dijon in a day it might be better to avoid all the hassle and go via Basel/Mulhouse, especially as the Lausanne - Dijon TGV runs 3 times a day only and has a 29€ mandatory reservation (+ SNCF tends to block sales if you don't go to Paris...). For example:

- RE Milano Centrale - Lugano 07:43 - 08:58

- IC21 Lugano - Basel SBB 09:02 - 12:56 (the connection is held for a few min if needed. There are trains every 30 min too)

- TER Basel SBB - Mulhouse 13:21 - 13:44

- TGV Mulhouse - Dijon 13:57 - 15:11 10€

Feel free to take a later TGV too and make a break in Lugano, Lucerne, Basel,...