The claim that the church placed All Saints on 1 November to overwrite Samhain is the standard explanation, it is old, and it is weaker than its currency suggests.
The sequence is documented. Rome kept a feast of all martyrs on 13 May, from the dedication of the Pantheon by Boniface IV around 609. Gregory III, between 731 and 741, dedicated an oratory in St Peter's to all saints, and the date of that dedication is itself uncertain in the sources. The 1 November observance is attested in Northumbria and Ireland by around 800, Alcuin, a Northumbrian, recommends it in correspondence, and in 835 Louis the Pious made it obligatory across the Frankish empire at the instance of Gregory IV.
The difficulty for the suppression theory is direction of travel. The November date appears in the insular and Frankish north and moves outward, rather than being imposed from Rome onto a Celtic fringe. James Frazer argued for the Samhain connection; Hutton argues the date is more plausibly a Germanic than a Celtic idea, and points out that the Irish church, which had Samhain on its doorstep, was not the source of the date.
It is also worth noting that the 13 May date has been argued to overlie the Roman Lemuria, a festival of restless spirits, which is the same kind of claim, made about a different date, and is no better evidenced.
What can be said is that a Christian feast of the dead and an older seasonal boundary ended up on the same days, and that the causal story usually told to explain it is not demonstrated.