The priest is meant to follow the text of the Eucharistic Prayer as printed, but the man in the pew needn’t be too concerned if he substitutes a printed word with a synonym from time to time (such as “happy” as opposed to “blessed”).
Bear in mind, that in the early liturgy, the Eucharistic Prayer was completely off the cuff. As time went on and priests were required to recite the Eucharistic Prayer as written, bishops still had the prerogative of making up their own. Now, we’re talking the first six or seven centuries of the Church, and I don’t mean to be an antiquarian, but ultimately this question pertains to licitness rather than validity.
That having been said, the rules that the Church has in place, today, are not nearly as free as they were once upon a time, and a priest today must operate in today’s Church, not in the Church of the first century.
It is precisely this sort of “I can do whatever I want” attitude on the part of the modern celebrant that appalls so many liturgically-informed Catholics, the sort of which post on this forum in great numbers. For a priest who blatantly disregards the directives and texts of the Church in favor of his own whims and predilections there can be no justification.
Regretably, those of us in the pews who care about the integrity of the liturgy have little practical choice but to suffer in silence the whims of egotistical celebrants and hope that the next generation of clergy hearken a brighter day for the liturgy.