I don't know why Barnabas, Julia, and Professor Stokes ever think of the harm they cause when they tamper with the timeline. They did so much damage in 1840 that I don't know where to begin.
Edith shouldn't have died in 1840. She originally died in 1897.
Angelique shouldn't have died in 1840 as a human. That means that she couldn't exist after 1840 which means that a lot of the things that Angelique did as a witch Post 1840 didn't happen.
Angelique would have been alive if it wasn't for Barnabas having a romance with Roxanne which caused Trask to be jealous of Barnabas. Plus it didn't help that he wanted revenge for his father's death.
Also, this means that Roxanne shouldn't have existed in the 1970 or 1970 PT time due to the fact that Roxanne died in 1840 which meant that she wouldn't have a PT counterpart for 1970.
Also, by Julia being in 1840 for a little while her presence created an 1841 PT counterpart. So there shouldn't have been a Julia 1841 PT counterpart.
I guess the only good thing about 1840 means that Carl Collins should be alive in 1897 and be married to Pansy Faye.
Barnabas and Julia should have just concentrated on their mission and not get so caught up in the lives of the 1840 Collins family & townsfolk. They changed way too much in 1840 that affected the future.
I think it would have been great if the producers & writers did a 1971 storyline that was about how so much got changed because of what they did in 1840.
Time travel stories are always tricky to do because it's so many details that writers have to remember in order not to cause conflict with stories that have already been written.