Lott made a joke about it when speaking at Thurmond's farewell dinner.Of them, John Bell Williams may never have formally joined the Republican Party but he did support Gerald Ford in 1976 and Ronald Reagan in 1980 (he died in 1983). https://en.wikipedia.org/wiki/John_Bell_Williams
William Colmer also may not have formally switched to the GOP but on his retirement he endorsed as his replacement his administrative assistant Tenet Lott, running as a Republican. (Lott of course later said that Thurmond should have been elected president in 1948...)
If we use "Dixiecrat" to mean "any southern segregationist in Congress" then signing the "Southern Manifesto" might be a criterion. https://en.wikipedia.org/wiki/Southern_Manifesto Of the Democrats who signed the Manifesto (there were also a few Republicans who did) , at least one signer other than Thurmond did later become a Republican: Armistead I. Selden, Jr.
After he retired, becoming a political zero.
In addition, at least one blatant segregationist who was elected to Congress as a Democrat after the Southern Manifesto was signed did switch to the Republicans--Albert Watson of South Carolina.
That's a good one. But one.
BTW, those who did not switch to the GOP nevertheless often discreetly supported GOP presidential candidates, so to say that they remained "loyal" to the Democrats to the end seems an exaggeration.
They caucused with the Democrats in Congress, they supported Democrats for every office except President. Saying "they remained loyal Democrats" is far less of an exaggeration than saying "they left the Democrats and hijacked the Republicans".