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Assuming that eq1 is E1[0,0].expr().expand() == 0, what about
sage: eq1.subs({diff(a.expr(),t): h*a.expr()})
3*h^2 - Lambda + 3*k/a(t)^2 == 0
Alternatively, you may want to keep h being a function of t:
sage: eq1.subs({diff(a.expr(),t): function('h')(t)*a.expr()})
3*h(t)^2 - Lambda + 3*k/a(t)^2 == 0
