(Reuters) -A surge of interest in bitcoin exchange-traded funds is prompting some investors to swap out holdings in gold-backed ETFs, although analysts and fund managers said they are unlikely to challenge bullion longer term. ETFs track an index, commodities, bonds or a basket of assets like an index fund. Since the Jan. 10 U.S. approval, two of the biggest new spot bitcoin ETFs, iShares Bitcoin Trust and Fidelity Wise Origin Bitcoin Fund, had accumulated $5.45 billion and $4.13 billion in assets respectively as of Feb. 14, LSEG Lipper data shows.