Pilgrimage is enjoined to the House of Bahá'u'lláh in Baghdad and the House of the Báb in Shíráz. (See Kitáb-i-Aqdas, Q29: "By pilgrimage to the sacred House, which is enjoined upon men, is intended both the
Most Great House in Baghdád and the House of the Primal Point in Shíráz; pilgrimage to either of
these Houses sufficeth. They may thus make pilgrimage to whichever lieth nearer to the place
where they reside.") Isn't the first house inaccessible, and the second one torn down? All pilgrims go to Haifa and Akká now, so how did this change occur?
Answer:
Abdu'l-Bahá designated the Shrine of Bahá'u'lláh at Bahji as a place of pilgrimage. Please refer to Note 54 (Aqdas p.191). Further details will be announced by the Universal House of Justice in due course. I'll share my personal understanding of the situation in relation to your question.
The point of attention in pilgrimage is not a building. It is the spot which is sacred because of its special background. When we read the procedure prescribed in the special Tablets for proper pilgrimage rites we notice that there is no mention of physically entering the building. All the rites are about how to prepare ourselves, how to approach the designated spot, how to circumambulate it and what verses be chanted or uttered. For example, Muslims that go to Mecca for pilgrimage do not enter the Ka'ba. They only circumambulate it.
We can easily imagine that in the future when millions of pilgrims will be going for pilgrimage every day cannot enter a small house and perform any rites. They can only circumambulate the spot. This is what is now taking place in Bahji when the number of visitors and pilgrims are large.
Another point to remember is that the houses built during nineteenth century in Iran and Baghdad were not constructed in a manner to remain intact for centuries. In order to maintain them, each one has to be rebuild several times. The house of the Báb in Shíráz had undergone several major restorations. It needed some major changes to be restored to its very original form as it was during the time of the Báb. What the enemies of the Faith did is what we had actually to do to some extent in order to restore it to its exact original form! Furthermore, construction engineers were of the opinion that sometimes later the whole building should be rebuilt in stronger structure so that it remains in good shape. We have all the blueprints, films, and photographs to rebuild that house. So you see that we have not lost that blessed house!
We have to be confident that what Bahá'u'lláh has ordained will come to pass. It is the Will of God and no human force can prevent or distort it. All that the enemies of the Faith have done is leave an ugly name for themselves that will make their future generations feel ashamed for centuries to come.